Add "now displaying" / "up next" preview pair to the frame header

The server now records exactly what was last sent to the device on
every device-facing render (/frame/image, /frame/advance, /frame/back,
and the global hold actions), persisted as Frame.last_displayed_image/
_at and served back via GET /api/frames/{id}/now-displaying. The
header thumbnail is split into that frozen "now displaying" snapshot
and the existing live "up next" re-render, with an arrow between them
-- so editing a layout shows the change immediately on the right while
the left stays exactly what's actually on the panel until the device's
next real wake.

server/app/routers/device.py

@@ -43,7 +43,7 @@ router = APIRouter()
 
 
 def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = None,
-                       as_png: bool = False) -> bytes:
+                       as_png: bool = False, capture_snapshot: bool = False) -> bytes | tuple[bytes, bytes]:
     """What an unclaimed or widget-less frame displays instead of real
     content -- instructions with a QR, rendered at 200 so the device
     treats it as a perfectly normal image and never error-loops. The
@@ -60,6 +60,7 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
             palette_rgb=frame.palette_rgb,
             manage=manage,
             as_png=as_png,
+            capture_snapshot=capture_snapshot,
         )
     if frame.owner_user_id is None:
         return render_placeholder(
@@ -68,6 +69,7 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
             palette_rgb=frame.palette_rgb,
             manage=manage,
             as_png=as_png,
+            capture_snapshot=capture_snapshot,
         )
     return render_placeholder(
         ["Almost there!", "Add a widget for this frame at", base],
@@ -76,11 +78,12 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
         palette_rgb=frame.palette_rgb,
         manage=manage,
         as_png=as_png,
+        capture_snapshot=capture_snapshot,
     )
 
 
 def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wake: bool,
-                    as_png: bool = False) -> bytes:
+                    as_png: bool = False, capture_snapshot: bool = False) -> bytes | tuple[bytes, bytes]:
     """The widget-system compositor: renders every widget on this frame
     into its own region (see app/grid.py for grid-cell -> pixel math),
     draws that widget's own optional border directly onto its region
@@ -112,11 +115,13 @@ def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wa
         regions, orientation=frame.orientation, palette_rgb=frame.palette_rgb,
         color_boost=frame.color_boost, contrast_boost=frame.contrast_boost,
         dither_strength=frame.dither_strength, manage=manage, as_png=as_png,
+        capture_snapshot=capture_snapshot,
     )
 
 
 def _render_frame_content(db: Session, frame: Frame, request: Request | None, manage: dict | None,
-                          is_normal_wake: bool, as_png: bool = False) -> bytes:
+                          is_normal_wake: bool, as_png: bool = False,
+                          capture_snapshot: bool = False) -> bytes | tuple[bytes, bytes]:
     """The top-level "what does this frame show right now" entry point.
     An unclaimed frame or one with no widgets yet gets the setup
     placeholder (needs `request` for its QR URLs -- only available on the
@@ -134,11 +139,11 @@ def _render_frame_content(db: Session, frame: Frame, request: Request | None, ma
         if request is None:
             return render_placeholder(
                 ["Almost there!"], orientation=frame.orientation, palette_rgb=frame.palette_rgb,
-                manage=manage, as_png=as_png,
+                manage=manage, as_png=as_png, capture_snapshot=capture_snapshot,
             )
-        return _setup_placeholder(frame, request, manage=manage, as_png=as_png)
+        return _setup_placeholder(frame, request, manage=manage, as_png=as_png, capture_snapshot=capture_snapshot)
 
-    return _render_widgets(db, frame, manage, is_normal_wake, as_png=as_png)
+    return _render_widgets(db, frame, manage, is_normal_wake, as_png=as_png, capture_snapshot=capture_snapshot)
 
 
 def render_frame_preview_png(db: Session, frame: Frame, request: Request) -> bytes:
@@ -245,6 +250,17 @@ def _manage_flag(request: Request) -> bool:
     return request.query_params.get("manage") == "1"
 
 
+def _record_last_displayed(db: Session, frame: Frame, png_snapshot: bytes) -> None:
+    """Persists exactly what a device-facing render just sent (upright
+    PNG, manage overlay included if present -- whatever's actually on the
+    panel) as this frame's "now displaying" snapshot, the frozen half of
+    the web UI's header preview pair (see api_frames.py's /now-displaying
+    endpoint and its always-live "up next" counterpart, /preview)."""
+    with frame_locked(db, frame.id) as locked:
+        locked.last_displayed_image = png_snapshot
+        locked.last_displayed_at = time.time()
+
+
 @router.get("/frame/image")
 def frame_image(
     request: Request, frame: Frame = Depends(require_device), db: Session = Depends(get_db)
@@ -262,9 +278,14 @@ def frame_image(
     ?manage=1 (the manage button) composites the manage overlay onto
     whatever this would have returned anyway -- see build_manage_content.
     This is also the "normal wake" that resets any calendar widget's
-    browse position back to today (see app/widgets/calendar.py)."""
+    browse position back to today (see app/widgets/calendar.py).
+
+    Also records what's returned as this frame's "now displaying"
+    snapshot (see _record_last_displayed) -- every other device-facing
+    render endpoint below does the same."""
     manage = build_manage_content(db, frame, request) if _manage_flag(request) else None
-    content = _render_frame_content(db, frame, request, manage, is_normal_wake=True)
+    content, snapshot = _render_frame_content(db, frame, request, manage, is_normal_wake=True, capture_snapshot=True)
+    _record_last_displayed(db, frame, snapshot)
     return Response(content=content, media_type="application/octet-stream")
 
 
@@ -277,7 +298,10 @@ def frame_advance(request: Request, frame: Frame = Depends(require_device), db:
     device's next-photo button."""
     _run_button_actions(db, frame, "next")
     manage = build_manage_content(db, frame, request) if _manage_flag(request) else None
-    content = _render_frame_content(db, frame, request=None, manage=manage, is_normal_wake=False)
+    content, snapshot = _render_frame_content(
+        db, frame, request=None, manage=manage, is_normal_wake=False, capture_snapshot=True
+    )
+    _record_last_displayed(db, frame, snapshot)
     return Response(content=content, media_type="application/octet-stream")
 
 
@@ -288,7 +312,10 @@ def frame_back(request: Request, frame: Frame = Depends(require_device), db: Ses
     with nothing to go back to. Used by the device's back-photo button."""
     _run_button_actions(db, frame, "back")
     manage = build_manage_content(db, frame, request) if _manage_flag(request) else None
-    content = _render_frame_content(db, frame, request=None, manage=manage, is_normal_wake=False)
+    content, snapshot = _render_frame_content(
+        db, frame, request=None, manage=manage, is_normal_wake=False, capture_snapshot=True
+    )
+    _record_last_displayed(db, frame, snapshot)
     return Response(content=content, media_type="application/octet-stream")
 
 
@@ -303,7 +330,10 @@ def frame_global_next(request: Request, frame: Frame = Depends(require_device),
     firmware/main/next_button.c for the short/long split."""
     _run_global_action(db, frame, "next")
     manage = build_manage_content(db, frame, request) if _manage_flag(request) else None
-    content = _render_frame_content(db, frame, request=None, manage=manage, is_normal_wake=False)
+    content, snapshot = _render_frame_content(
+        db, frame, request=None, manage=manage, is_normal_wake=False, capture_snapshot=True
+    )
+    _record_last_displayed(db, frame, snapshot)
     return Response(content=content, media_type="application/octet-stream")
 
 
@@ -312,7 +342,10 @@ def frame_global_back(request: Request, frame: Frame = Depends(require_device),
     """The mirror of /frame/global-next, for a held BACK button."""
     _run_global_action(db, frame, "back")
     manage = build_manage_content(db, frame, request) if _manage_flag(request) else None
-    content = _render_frame_content(db, frame, request=None, manage=manage, is_normal_wake=False)
+    content, snapshot = _render_frame_content(
+        db, frame, request=None, manage=manage, is_normal_wake=False, capture_snapshot=True
+    )
+    _record_last_displayed(db, frame, snapshot)
     return Response(content=content, media_type="application/octet-stream")