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/frame_pages.py

@@ -12,6 +12,7 @@ from sqlalchemy.orm import Session
 
 from ..auth import can_view_frame, current_user
 from ..db import get_db
+from ..image_pipeline import DEFAULT_PALETTE_RGB, PALETTE_LABELS, palette_to_hex
 from ..models import Frame
 from ..quiet_hours import ALL_TIMEZONES
 from .common import shell_context
@@ -40,7 +41,11 @@ def frame_photos_page(frame_id: int, request: Request, db: Session = Depends(get
 @router.get("/frames/{frame_id}/config", response_class=HTMLResponse)
 def frame_config_page(frame_id: int, request: Request, db: Session = Depends(get_db)):
     return _frame_page(
-        request, db, frame_id, "frame_config.html", "config", timezones=ALL_TIMEZONES
+        request, db, frame_id, "frame_config.html", "config",
+        timezones=ALL_TIMEZONES,
+        palette_labels=PALETTE_LABELS,
+        default_palette_rgb=DEFAULT_PALETTE_RGB,
+        palette_to_hex=palette_to_hex,
     )