Widget system Phase 4b: per-widget gear-icon config dialogs

Replaces the Photos/Calendar/Whiteboard tabs with a single Layout page
(now the frame's landing route) where each widget gets a gear icon
opening a dialog scoped to that specific widget's own settings. This
was the missing piece for genuinely independent same-type widgets --
"the Calendar tab" never made sense once a frame could hold more than
one calendar widget with different settings.

Data layer: FrameCalendar re-keyed from frame_id to widget_id, so each
calendar widget has its own independent included-calendars set. The
rekey runs as an unconditional post-startup step (like the existing
widget backfill), not a numbered migration -- it depends on calendar
widgets already existing, which themselves come from that same
backfill step, not from schema migration. Registering it as a numbered
migration would have run it first during a real upgrade, silently
dropping every row; caught by a new test that exercises the raw-SQL
upgrade path instead of the fresh-install create_all() shortcut every
other migration test takes.

API layer: every endpoint that used to assume "the frame's widget of
this type" (photo queue/thumbnail/preview, calendar select/color/
tasks/weather, whiteboard source/browse/preview) moved into
api_widgets.py under /api/frames/{id}/widgets/{widget_id}/..., with a
new require_widget_view/control dependency pair mirroring the existing
frame-level ones. Device status (battery/last-seen/firmware) got its
own frame-level /status endpoint, split out of the old photo-specific
/queue it used to piggyback on -- fixes the status bar going silently
blank on any frame without a photo widget.

UI layer: each widget type's existing settings markup/JS was ported
into a dialog partial + an explicit init/close function pair (the
content is now fetched and injected on demand, not loaded at page load
time). window.FRAME_API is repointed to the open dialog's widget-scoped
API base for its duration and restored on close; a separate
window.FRAME_BASE_API stays stable for the always-present header/
status-bar scripts.

Caught during manual browser testing: the consolidated config-save
endpoint initially expected a JSON body while the copied-over dialog JS
posts form-urlencoded data (the old convention) -- fixed to match, with
new HTTP-level test coverage that would have caught it immediately.

server/app/static/settings.js

@@ -1,7 +1,7 @@
 // Settings page: "Discover calendars" against the CalDAV account already
-// saved on this form (same idiom as frame_photos.js's Load Albums using
-// the frame's already-saved Immich creds) -- so this only works after
-// the CalDAV URL/username/password have been saved once.
+// saved on this form (same idiom as widget_dialog_photos.js's Load
+// Albums using the frame's already-saved Immich creds) -- so this only
+// works after the CalDAV URL/username/password have been saved once.
 
 // Hides the dedicated WebDAV username/password fields while "reuse my
 // CalDAV creds" is checked -- they'd be ignored server-side anyway (see