fix(mobile): replace 404 media library error with session-required state and device gallery fallback

The Postiz public API v1 does not expose a media listing endpoint.
Switch URL to the correct internal path (/media?page=0&search=), handle
the resulting 401 with a dedicated lock-icon state, and wire the existing
device gallery picker as an onPickFromDevice fallback so the modal stays
usable.

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

artifacts/postiz-mobile/app/(tabs)/compose.tsx

@@ -767,6 +767,7 @@ export default function ComposeScreen() {
         workspaces={workspaces}
         maxSelect={MAX_IMAGES - mediaItems.length}
         onClose={() => setShowMediaLibrary(false)}
+        onPickFromDevice={() => { setShowMediaLibrary(false); pickImage(); }}
         onSelect={(items: LibraryMediaItem[]) => {
           setMediaItems((prev) =>
             [