feat: band NC folder config, fix watcher event filter, add light/dark theme

- Add PATCH /bands/{id} endpoint for admins to update nc_folder_path
- Add band NC scan folder UI panel with inline edit
- Fix watcher: use activity type field (not human-readable subject) for upload detection
- Reorder watcher filters: audio extension check first, then band path, then type
- Add dark/light theme toggle using GitHub Primer-inspired CSS custom properties
- All inline styles migrated to CSS variables for theme-awareness

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/src/rehearsalhub/schemas/band.py

@@ -21,6 +21,10 @@ class BandCreate(BaseModel):
     nc_base_path: str | None = None  # e.g. "Bands/MyBand/" — defaults to "bands/{slug}/"
 
 
+class BandUpdate(BaseModel):
+    nc_folder_path: str | None = None  # update the Nextcloud base folder for scans
+
+
 class BandRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
     id: uuid.UUID