Add Docker update support via Watchtower integration

Add web-based Docker container updates using Watchtower HTTP API.
When configured with WATCHTOWER_API_URL and WATCHTOWER_API_TOKEN
environment variables, administrators can trigger container updates
from the Update Manager page.

Features:
- WatchtowerClient service for Watchtower HTTP API communication
- Docker update progress page with animated Docker whale logo
- Real-time step tracking: Trigger, Pull, Stop, Restart, Health Check, Verify
- CSP-compatible progress bar using CSS classes
- Translated UI strings via Stimulus values
- Health endpoint polling to detect container restart
- Watchtower setup documentation for Docker installations
- WatchtowerClient made nullable for non-Docker installations
- Unit tests for WatchtowerClient

.env

@@ -76,6 +76,12 @@ DISABLE_BACKUP_RESTORE=1
 # When enabled, users must confirm their password before downloading.
 DISABLE_BACKUP_DOWNLOAD=1
 
+# Watchtower integration for Docker-based updates.
+# Set these to enable one-click updates via the Update Manager UI.
+# See https://containrrr.dev/watchtower/ for Watchtower setup.
+WATCHTOWER_API_URL=
+WATCHTOWER_API_TOKEN=
+
 ###################################################################################
 # SAML Single sign on-settings
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