feat(web): add system announcement banner and maintenance mode

Add two operator-controlled, startup-time settings:

- SYSTEM_ANNOUNCEMENT: non-dismissable Markdown banner rendered above the
  network announcement on every page (navbar -> system -> network).
- SYSTEM_MAINTENANCE: when enabled, forces all feature flags off so the nav
  collapses to Home, hides the profile menu, and the SPA renders a maintenance
  page for every route. The maintenance page makes no API calls, so the API
  and database can be offline while the web component keeps running.

CLI exposes --system-announcement and tri-state --system-maintenance; the bool
falls back to pydantic settings to parse SYSTEM_MAINTENANCE reliably from env.
Adds i18n strings (en/nl), tests, and docs.

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

.env.example

@@ -496,6 +496,19 @@ NETWORK_WELCOME_TEXT=
 # Example: **Maintenance** scheduled for Saturday — see [details](https://example.com)
 NETWORK_ANNOUNCEMENT=
 
+# System announcement banner (optional, Markdown supported)
+# Non-dismissable banner shown above the network announcement on every page,
+# for important system notices (downtime, maintenance windows, alerts).
+# Stays visible until unset and the web service is restarted. Empty = no banner.
+SYSTEM_ANNOUNCEMENT=
+
+# Maintenance mode (default: false)
+# When true, disables almost all site functionality: the nav shows only Home,
+# the user/profile menu is hidden, and every page renders a "Site Under
+# Maintenance" notice. No backend API calls are made, so the API/database can
+# be offline while the web component keeps running. Requires a web restart.
+SYSTEM_MAINTENANCE=false
+
 # -------------------
 # Feature Flags
 # -------------------