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/en.ts

@@ -485,6 +485,12 @@ export const enTranslation = {
     userMgmtPassword: 'Password',
     userMgmtPasswordEditHint: 'Enter a new password to update it.',
     userMgmtRoleAdmin: 'Admin',
+    userMgmtLibraries: 'Libraries',
+    userMgmtLibrariesAdminHint: 'Admin users automatically have access to all libraries.',
+    userMgmtLibrariesEmpty: 'No libraries available on this server.',
+    userMgmtLibrariesValidation: 'Select at least one library.',
+    userMgmtLibrariesUpdateError: 'User saved, but library assignment failed',
+    userMgmtNoLibraries: 'No libraries assigned',
     userMgmtSave: 'Save',
     userMgmtCancel: 'Cancel',
     userMgmtDelete: 'Delete',