feat: sync DB prefs, update history tab, configurable dashboard shortcuts

- auth store fetchMe(): sync theme/sidebar_position/lang from DB to localStorage+stores on login/refresh
- profilePage setters: PATCH /api/auth/preferences on every preference change
- updatesPage: add history tab (GET /api/updates/history) with job list, click to view output
- dashboardPage: load shortcuts from settings API, fall back to defaults if none configured
- settingsPage: new Raccourcis tab to add/remove/configure dashboard shortcuts (saved as JSON)
- settings.go: expose dashboard_shortcuts in publicSettings for GET/PUT access
- pages.css: add .history-table, .shortcut-row styles

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

backend/internal/api/settings.go

@@ -32,6 +32,7 @@ var publicSettings = []string{
 	"proxmox_url",
 	"ssh_host",
 	"ssh_username",
+	"dashboard_shortcuts",
 }
 
 // paramètres sensibles : modifiables en écriture seule, stockés chiffrés.