feat: add granular auto-backup scheduling and timezone support

Add UI controls for configuring auto-backup schedule with hour, day of
week, and day of month pickers. The hour picker respects the user's
12h/24h time format preference from settings.

Add TZ environment variable support via docker-compose so the container
runs in the configured timezone. The timezone is passed to node-cron for
accurate scheduling and exposed via the API so the UI displays it.

Fix SQLite UTC timestamp handling by appending Z suffix to all timestamps
sent to the client, ensuring proper timezone conversion in the browser.

Made-with: Cursor

client/src/types.ts

@@ -279,6 +279,7 @@ export interface AppConfig {
   oidc_display_name?: string
   has_maps_key?: boolean
   allowed_file_types?: string
+  timezone?: string
 }
 
 // Translation function type