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/README.md

@@ -153,7 +153,10 @@ Pages: `/` (routing hub), `/setup`, `/login`, `/claim`, `/settings`,
   `quiet_hours_*` + `timezone` (a pure server-side decision shaping
   what `refresh_interval_s` gets handed to the device),
   `firmware_update_repo_url`, `firmware_auto_update`,
-  `battery_alert_threshold_pct` -- percent, or `-1`/blank to disable).
+  `battery_alert_threshold_pct` -- percent, or `-1`/blank to disable --,
+  `palette` -- exactly 6 `#rrggbb` values in black/white/yellow/red/
+  blue/green order --, `palette_reset` -- `true` clears back to the
+  default palette).
 - `POST .../take-control` -- always succeeds for a linked user.
 - `GET  .../stats`, `GET .../battery-log`, `GET .../thumbnail/{asset_id}`.
 - `POST .../firmware` (manual .bin upload, esp_app_desc_t-validated),
@@ -193,10 +196,14 @@ Pages: `/` (routing hub), `/setup`, `/login`, `/claim`, `/settings`,
   to photos this frame is actually showing or has queued, not any
   Immich asset ID someone might guess -- a second layer a leaked device
   token alone wouldn't bypass.
-- The 6-color palette RGB values in `app/image_pipeline.py` are
-  approximations, not measured values (Waveshare doesn't publish exact
-  color primaries for this panel) -- tune them once you can compare a
-  rendered test image against the real panel.
+- The 6-color palette RGB values in `app/image_pipeline.py`
+  (`DEFAULT_PALETTE_RGB`) are approximations, not measured values
+  (Waveshare doesn't publish exact color primaries for this panel).
+  Each frame's Configuration tab has an **Advanced configuration**
+  section (collapsed by default) with a color picker per ink color --
+  tune them once you can compare a rendered photo against the real
+  panel, and "Reset to defaults" to go back. Different panel units can
+  vary enough to be worth calibrating per frame.
 
 ## Deploying a pre-built image