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/main.py

@@ -231,6 +231,7 @@ def _migrate():
             ("cache_months",     "ALTER TABLE user_settings ADD COLUMN cache_months INTEGER DEFAULT 3"),
             ("month_view_paged", "ALTER TABLE user_settings ADD COLUMN month_view_paged BOOLEAN DEFAULT 0"),
             ("sync_flags",       "ALTER TABLE user_settings ADD COLUMN sync_flags TEXT"),
+            ("surface_color",    "ALTER TABLE user_settings ADD COLUMN surface_color VARCHAR(7)"),
         ):
             try:
                 conn.execute(text(ddl))