Add a live preview thumbnail next to the frame name

Small header thumbnail showing exactly what the frame is currently
displaying -- same widget compositor /frame/image uses, handed back as
a plain PNG (image_pipeline.render_panel/render_placeholder gain an
as_png option) instead of packed native-panel bytes. New session-authed
GET /api/frames/{id}/preview exposes it; click-to-refresh plus a slow
60s poll on the frame header so it doesn't hammer Immich/calendar
sources just for a header thumbnail.

server/app/routers/device.py

@@ -45,7 +45,8 @@ logger = logging.getLogger(__name__)
 router = APIRouter()
 
 
-def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = None) -> bytes:
+def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = None,
+                       as_png: bool = False) -> 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
@@ -61,6 +62,7 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
             orientation=frame.orientation,
             palette_rgb=frame.palette_rgb,
             manage=manage,
+            as_png=as_png,
         )
     if frame.owner_user_id is None:
         return render_placeholder(
@@ -68,6 +70,7 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
             orientation=frame.orientation,
             palette_rgb=frame.palette_rgb,
             manage=manage,
+            as_png=as_png,
         )
     return render_placeholder(
         ["Almost there!", "Add a widget for this frame at", base],
@@ -75,10 +78,12 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
         orientation=frame.orientation,
         palette_rgb=frame.palette_rgb,
         manage=manage,
+        as_png=as_png,
     )
 
 
-def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wake: bool) -> bytes:
+def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wake: bool,
+                    as_png: bool = False) -> bytes:
     """The widget-system compositor: renders every widget on this frame
     into its own region (see app/grid.py for grid-cell -> pixel math) and
     hands the results to image_pipeline.render_panel for the single
@@ -102,12 +107,12 @@ def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wa
     return render_panel(
         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,
+        dither_strength=frame.dither_strength, manage=manage, as_png=as_png,
     )
 
 
 def _render_frame_content(db: Session, frame: Frame, request: Request | None, manage: dict | None,
-                          is_normal_wake: bool) -> bytes:
+                          is_normal_wake: bool, as_png: bool = False) -> 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
@@ -124,11 +129,23 @@ def _render_frame_content(db: Session, frame: Frame, request: Request | None, ma
     if not has_widgets:
         if request is None:
             return render_placeholder(
-                ["Almost there!"], orientation=frame.orientation, palette_rgb=frame.palette_rgb, manage=manage
+                ["Almost there!"], orientation=frame.orientation, palette_rgb=frame.palette_rgb,
+                manage=manage, as_png=as_png,
             )
-        return _setup_placeholder(frame, request, manage=manage)
+        return _setup_placeholder(frame, request, manage=manage, as_png=as_png)
 
-    return _render_widgets(db, frame, manage, is_normal_wake)
+    return _render_widgets(db, frame, manage, is_normal_wake, as_png=as_png)
+
+
+def render_frame_preview_png(db: Session, frame: Frame, request: Request) -> bytes:
+    """The web UI's live "how it's displaying" thumbnail (see
+    routers/api_frames.py's /preview endpoint) -- same compositor
+    /frame/image uses, just handed back as a small upright PNG instead of
+    packed native-panel bytes. Exported from here (rather than
+    duplicated) since this module already owns the full widget-
+    compositing pipeline; nothing about the /frame/* paths themselves
+    changes."""
+    return _render_frame_content(db, frame, request, manage=None, is_normal_wake=True, as_png=True)
 
 
 def _run_button_actions(db: Session, frame: Frame, button: str) -> None: