Add configurable surface/sidebar colour + clearer sync-icon state

- new user_settings.surface_color (nullable, device-local default, not synced):
  drives the web sidebar / top bar / surfaces; NULL derives from bg_color as
  before. Added to schema, migration, GET/PUT, NULLABLE_OVERRIDES, DEFAULT_SYNC.
- web: surface colour row in the settings table; applyTheme derives the whole
  surface family from it
- web: per-row sync icon now has real styling — ON shows the primary-tinted
  glyph on a pill, OFF shows a slashed, muted glyph (was previously unstyled,
  so synced/not-synced looked identical)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/models.py

@@ -100,6 +100,9 @@ class UserSettings(Base):
     text_color = Column(String(7), nullable=True)   # Override für --text-1 (NULL = nutze text_contrast)
     line_color = Column(String(7), nullable=True)   # Override für --border  (NULL = nutze line_contrast)
     bg_color   = Column(String(7), nullable=True)   # Override für --bg-app  (NULL = Default)
+    # Surface/sidebar/topbar colour (web sidebar + top bar, iOS top bar).
+    # NULL = derive from bg_color. Device-local by default (not synced).
+    surface_color = Column(String(7), nullable=True)
     # How this user's private events appear to other group members:
     # 'hidden' = invisible, 'busy' = anonymous busy block (default).
     private_event_visibility = Column(String(10), default="busy")