Server web UI: modern themeable layout with dark mode, timezone moved into config

Replaces the ad-hoc inline styling with a shared base template driven by
CSS custom properties (light/dark palettes), a card-based two-column
layout, and a persistent dark-mode toggle. Also moves quiet-hours'
timezone from the container's TZ env var into a proper web UI setting
(zoneinfo-backed), so it no longer needs a docker-compose.yml edit and
restart to change.

server/app/config.py

@@ -27,15 +27,20 @@ class FrameConfig(BaseModel):
     order: str = "sequential"  # or "shuffle"
     refresh_interval_s: int = 3600
     # Quiet hours: no point waking the device overnight just to swap a
-    # photo nobody's looking at. Times are "HH:MM" in the server's local
-    # timezone (see docker-compose.yml.example's TZ note) and may wrap
-    # past midnight (e.g. start=22:00, end=07:00). Purely a server-side
-    # decision -- the device is unaware, it just gets told a longer
-    # refresh_interval_s by GET /frame/config while quiet hours are in
-    # effect (see main.py's _effective_refresh_interval_s).
+    # photo nobody's looking at. Times are "HH:MM" interpreted in
+    # `timezone` below and may wrap past midnight (e.g. start=22:00,
+    # end=07:00). Purely a server-side decision -- the device is unaware,
+    # it just gets told a longer refresh_interval_s by GET /frame/config
+    # while quiet hours are in effect (see main.py's
+    # _effective_refresh_interval_s).
     quiet_hours_enabled: bool = False
     quiet_hours_start: str = "22:00"
     quiet_hours_end: str = "07:00"
+    # IANA zone name (e.g. "America/New_York") quiet_hours_start/end are
+    # interpreted in. Set from the web UI rather than the container's TZ
+    # environment variable, so it survives container recreation and
+    # doesn't need a docker-compose.yml edit to change.
+    timezone: str = "UTC"
     smart_crop_faces: bool = True
     # How the physical frame is hung: landscape (native), portrait,
     # landscape_flipped, portrait_flipped. Purely a server-side render