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/templates/_widget_dialog_photos.html

@@ -0,0 +1,55 @@
+<h2 class="dialog-title">Photos widget</h2>
+
+<section class="card">
+  <h2 class="card-title">Album</h2>
+  <form id="photos-form">
+    <label>Album
+      <select id="album_id">
+        {% if photo_cfg and photo_cfg.album_id %}<option value="{{ photo_cfg.album_id }}" selected>(current selection -- Load Albums to change)</option>{% endif %}
+      </select>
+    </label>
+    <label>Upcoming photos to show
+      <select id="queue_target_len">
+        {% for n in [5, 10, 15, 20, 25, 30, 40, 50] %}
+          <option value="{{ n }}" {% if photo_cfg and photo_cfg.queue_target_len == n %}selected{% endif %}>{{ n }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <label>Order
+      <select id="order">
+        <option value="sequential" {% if not photo_cfg or photo_cfg.order == "sequential" %}selected{% endif %}>Sequential</option>
+        <option value="shuffle" {% if photo_cfg and photo_cfg.order == "shuffle" %}selected{% endif %}>Shuffle</option>
+      </select>
+    </label>
+    <label>Display mode
+      <select id="display_mode">
+        {% for mode, label in display_mode_labels.items() %}
+          <option value="{{ mode }}" {% if photo_cfg and photo_cfg.display_mode == mode %}selected{% endif %}>{{ label }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <p class="sub" style="margin-top: 8px;">How a photo's aspect ratio
+      is reconciled with the panel's: <strong>Crop to fill</strong>
+      trims the excess; <strong>Crop to faces</strong> does the same
+      but shifts the crop to keep people on screen; <strong>Stretch to
+      fill</strong> fills the panel exactly without cropping (photos
+      not matching the panel's aspect ratio look stretched);
+      <strong>Shrink to fit</strong> shows the whole photo, letterboxed
+      if needed.</p>
+    <button type="button" class="secondary" id="load-albums">Load Albums</button>
+    <button type="submit">Save</button>
+  </form>
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Now displaying</h2>
+  <div id="current-thumb"><p class="sub">Loading...</p></div>
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Upcoming</h2>
+  <p class="sub">Drag a photo to reorder (on touch, hold briefly first so a
+    normal scroll still works), "Show next" to jump it to the front, or
+    the &times; to remove it from rotation entirely.</p>
+  <div id="upcoming-grid" class="photo-grid"></div>
+</section>