feat(users): per-user library assignment + themed confirm modal (#222)

Adds Navidrome library assignment to the User Management settings
panel: GET /api/library + PUT /api/user/{id}/library wired through
new Rust commands. UserForm gets a checkbox picker (hidden for
admins, who auto-receive all libraries server-side) with inline
validation. User rows compacted to a single clickable line with
hover highlight; native confirm() replaced by a portal-based
ConfirmModal (theme-aware, ESC/Enter, vertically centered).

Co-authored-by: Psychotoxical <dev@psysonic.app>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/de.ts

@@ -483,6 +483,12 @@ export const deTranslation = {
     userMgmtPassword: 'Passwort',
     userMgmtPasswordEditHint: 'Neues Passwort eingeben, um es zu ändern.',
     userMgmtRoleAdmin: 'Admin',
+    userMgmtLibraries: 'Bibliotheken',
+    userMgmtLibrariesAdminHint: 'Admin-Benutzer haben automatisch Zugriff auf alle Bibliotheken.',
+    userMgmtLibrariesEmpty: 'Keine Bibliotheken auf diesem Server vorhanden.',
+    userMgmtLibrariesValidation: 'Mindestens eine Bibliothek auswählen.',
+    userMgmtLibrariesUpdateError: 'Benutzer gespeichert, aber Bibliothekszuweisung fehlgeschlagen',
+    userMgmtNoLibraries: 'Keine Bibliotheken zugewiesen',
     userMgmtSave: 'Speichern',
     userMgmtCancel: 'Abbrechen',
     userMgmtDelete: 'Löschen',