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/static/theme.css

@@ -168,6 +168,30 @@ summary.card-title { cursor: pointer; margin-bottom: 0; }
 details.card[open] summary.card-title { margin-bottom: 14px; }
 details.card .sub { margin-top: 8px; }
 
+.palette-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(110px, 1fr));
+  gap: 12px;
+  margin-top: 14px;
+}
+.palette-swatch {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin-top: 0;
+  font-size: 13px;
+  font-weight: 500;
+  color: var(--text);
+}
+.palette-swatch input[type="color"] {
+  width: 34px;
+  height: 34px;
+  padding: 2px;
+  margin-top: 0;
+  border-radius: 8px;
+  cursor: pointer;
+}
+
 .card {
   background: var(--surface);
   border: 1px solid var(--border);