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/frame_config.html

@@ -158,19 +158,25 @@
 
       <section class="card">
         <h2 class="card-title">Preview</h2>
-        <p class="sub">The current photo, and exactly how it renders on the
-          panel with this frame's saved settings above.</p>
-        <div class="preview-compare">
-          <div class="preview-pane">
-            <p class="sub">Now displaying</p>
-            <img class="preview-img" id="preview-original" alt="Original photo">
+        {% if photo_widget_id %}
+          <p class="sub">The current photo (from this frame's photo widget), and
+            exactly how it renders on the panel with this frame's saved settings
+            above.</p>
+          <div class="preview-compare">
+            <div class="preview-pane">
+              <p class="sub">Now displaying</p>
+              <img class="preview-img" id="preview-original" alt="Original photo">
+            </div>
+            <div class="preview-pane">
+              <p class="sub">How it will look on the frame</p>
+              <img class="preview-img" id="preview-rendered" alt="Rendered preview">
+            </div>
           </div>
-          <div class="preview-pane">
-            <p class="sub">How it will look on the frame</p>
-            <img class="preview-img" id="preview-rendered" alt="Rendered preview">
-          </div>
-        </div>
-        <button type="button" class="secondary" id="preview-refresh">Refresh preview</button>
+          <button type="button" class="secondary" id="preview-refresh">Refresh preview</button>
+        {% else %}
+          <p class="sub">This frame doesn't have a photo widget to preview
+            against yet -- add one from the Layout tab.</p>
+        {% endif %}
       </section>
     </div>
   </div>
@@ -180,8 +186,9 @@
 
 {% block scripts %}
   <script>
-    window.FRAME_API = {{ ("/api/frames/" ~ frame.id) | tojson }};
+    window.FRAME_BASE_API = window.FRAME_API = {{ ("/api/frames/" ~ frame.id) | tojson }};
     window.DEFAULT_PALETTE_HEX = {{ palette_to_hex(default_palette_rgb) | tojson }};
+    window.PHOTO_WIDGET_PREVIEW_API = {{ (("/api/frames/" ~ frame.id ~ "/widgets/" ~ photo_widget_id) | tojson) if photo_widget_id else "null" }};
   </script>
   <script src="/static/device_status_bar.js"></script>
   <script src="/static/frame_header.js"></script>