Per-frame palette calibration + sidebar battery indicator

Advanced configuration (Configuration tab, collapsed <details> section):
a color picker per ink color (black/white/yellow/red/blue/green),
overriding image_pipeline.DEFAULT_PALETTE_RGB for that frame's actual
panel -- different units can vary enough from the documented
approximations to be worth calibrating once you can compare a rendered
photo against the real hardware. Stored as Frame.palette_rgb (NULL =
default, schema migration v4), threaded through render_frame/
render_placeholder/_quantize_and_pack (which now builds the PIL palette
image per call instead of once at import) so both photos and the
unclaimed/unconfigured placeholder screen respect it. "Reset to
defaults" clears back to NULL. Config-save validates exactly 6 #rrggbb
values, rejecting anything else with a 400.

Also: each frame's sidebar entry now shows its last-reported battery
percent (🔋NN%) next to the name, using the frame_dot's existing
recently-seen indicator conventions -- silent when never reported
(mains-only frames, or before the first report), matching how battery
is hidden everywhere else it's not applicable.

Verified against the same live-shaped database as the SMTP work: the
v3->v4 migration, save/reload/reset round trip through the real HTTP
route, an actual rendered image using a custom palette (confirmed via
its packed panel-code bytes), input validation, and the sidebar badge
against real battery data -- plus the standing legacy-device curl suite.

server/app/templates/frame_config.html

@@ -112,6 +112,26 @@
           disable. Needs SMTP configured by an admin.</p>
         <button type="button" class="secondary" id="battery-alert-save">Save</button>
       </section>
+
+      <details class="card">
+        <summary class="card-title">Advanced configuration</summary>
+        <p class="sub">Color-quantization values used when dithering photos
+          for this panel -- approximations by default, since exact primaries
+          aren't published. Tune them by comparing a rendered photo against
+          the physical panel; different panel units can vary enough to be
+          worth calibrating per frame.</p>
+        <div class="palette-grid">
+          {% set current_hex = palette_to_hex(frame.palette_rgb or default_palette_rgb) %}
+          {% for label in palette_labels %}
+            <label class="palette-swatch">
+              <input type="color" id="palette_{{ loop.index0 }}" value="{{ current_hex[loop.index0] }}">
+              {{ label }}
+            </label>
+          {% endfor %}
+        </div>
+        <button type="button" class="secondary" id="palette-save">Save</button>
+        <button type="button" class="secondary" id="palette-reset">Reset to defaults</button>
+      </details>
     </div>
   </div>
 
@@ -119,6 +139,9 @@
 {% endblock %}
 
 {% block scripts %}
-  <script>window.FRAME_API = {{ ("/api/frames/" ~ frame.id) | tojson }};</script>
+  <script>
+    window.FRAME_API = {{ ("/api/frames/" ~ frame.id) | tojson }};
+    window.DEFAULT_PALETTE_HEX = {{ palette_to_hex(default_palette_rgb) | tojson }};
+  </script>
   <script src="/static/frame_config.js"></script>
 {% endblock %}