Move button actions to per-widget config, add hold-for-global-action

Next/back button assignment moves from a frame-level "Button
assignments" card into each widget's own gear-icon dialog, prefilled
with a sane default at creation (photos/calendar -> advance/back,
whiteboard/weather -> check_now, others -> none). At most one binding
per (widget, button) now -- cross-widget execution order never
mattered since each widget's action only touches its own state.

New firmware capability: holding NEXT or BACK past a configurable
duration (min 3s, server-side default) triggers a frame-wide action
instead of the per-widget short-press one -- cycling saved layouts,
refreshing all widgets, or freezing/unfreezing every photo widget (see
app/global_actions.py). Firmware next/back checks gain the same
hold-duration polling the combo button already had; the threshold
comes from the previous wake's /frame/config fetch (persisted in NVS),
since this wake's button decision happens before that request.

Not done here: firmware/version.txt is intentionally left unbumped --
this hasn't been built or hardware-tested (no ESP-IDF toolchain in this
environment), so no firmware release build should be triggered yet.

server/app/widgets/__init__.py

@@ -36,6 +36,7 @@ Each module in this package exposes:
 
 from __future__ import annotations
 
+from ..models import FrameButtonAction
 from . import battery, calendar, photos, static_image, tasks, text, weather, whiteboard
 
 WIDGET_TYPES = {
@@ -48,3 +49,31 @@ WIDGET_TYPES = {
     "weather": weather,
     "battery": battery,
 }
+
+
+def default_button_actions(frame_id: int, widget_id: int, widget_type: str) -> list[FrameButtonAction]:
+    """NEXT/BACK -> whatever this widget's own advance/back concept is.
+    Called both when backfilling pre-widget-system frames (see
+    app/migration.py) and when a widget is newly created (see
+    routers/api_widgets.py's api_widget_create) -- a widget is never left
+    without a sane starting binding, so the physical buttons always do
+    something reasonable for it until someone deliberately reassigns
+    them in that widget's own config dialog."""
+    if widget_type == "whiteboard":
+        # No real "next"/"back" concept for a static board -- both
+        # buttons mean "check now".
+        return [
+            FrameButtonAction(frame_id=frame_id, button="next", widget_id=widget_id, action="check_now"),
+            FrameButtonAction(frame_id=frame_id, button="back", widget_id=widget_id, action="check_now"),
+        ]
+    if widget_type == "weather":
+        return [
+            FrameButtonAction(frame_id=frame_id, button="next", widget_id=widget_id, action="check_now"),
+            FrameButtonAction(frame_id=frame_id, button="back", widget_id=widget_id, action="check_now"),
+        ]
+    if widget_type in ("photos", "calendar"):
+        return [
+            FrameButtonAction(frame_id=frame_id, button="next", widget_id=widget_id, action="advance"),
+            FrameButtonAction(frame_id=frame_id, button="back", widget_id=widget_id, action="back"),
+        ]
+    return []