Add Audible region config and user password change modal

Implements configurable Audible region selection in setup and admin settings, affecting all Audible API calls and triggering data refresh on change. Adds a user-facing 'Change Password' modal in the header for local users, moving password change from admin-only to all local users via a new /api/auth/change-password endpoint. Updates documentation, API routes, and context to support these features, and removes the old admin-only password change flow.

documentation/frontend/components.md

@@ -23,7 +23,7 @@ src/components/
 ## Key Components
 
 **Layout**
-- **Header** - Top nav, search input, user menu
+- **Header** ✅ - Top nav, search input, user menu with "Change Password" option (local users only), logout
 - **Sidebar** - Admin side nav
 - **Footer** - Version, links
 
@@ -46,6 +46,7 @@ src/components/
 - **Select** - Custom styling, search/filter
 - **Modal** ✅ - Dialog overlay with backdrop, sizes (sm/md/lg/xl/full), ESC to close, body scroll lock
 - **ConfirmModal** ✅ - Confirmation dialog with customizable title, message, buttons, loading state, and variant (primary/danger)
+- **ChangePasswordModal** ✅ - Password change form for local users. Three fields (current, new, confirm), real-time validation, success/error states, auto-closes on success. Only accessible to users with `authProvider='local'`
 - **Pagination** ✅ - Traditional page navigation with prev/next buttons, smart ellipsis (shows 1...4 5 6...10)
 - **StickyPagination** ✅ - Minimal floating pill at bottom center with prev/next arrows, quick jump input, section label. Shows/hides based on section visibility (IntersectionObserver). Rounded-full design, backdrop blur, subtle shadow, auto-scroll on page change