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/routers/device.py

@@ -54,16 +54,19 @@ def _setup_placeholder(frame: Frame, request: Request) -> bytes:
             ["This frame isn't claimed yet", "Scan to link it to your account:"],
             qr_url=claim_url,
             orientation=frame.orientation,
+            palette_rgb=frame.palette_rgb,
         )
     if frame.owner_user_id is None:
         return render_placeholder(
             ["Almost there!", f"Open {base} to finish setting up this frame."],
             orientation=frame.orientation,
+            palette_rgb=frame.palette_rgb,
         )
     return render_placeholder(
         ["Almost there!", "Pick an album for this frame:", base],
         qr_url=base,
         orientation=frame.orientation,
+        palette_rgb=frame.palette_rgb,
     )