Add multi-photo bulk upload to the Android app

Pick or share several photos at once and post them in one batch, still
against the existing single-image /upload endpoint — no server change.

- Unify the post screen on a list model (PickedItem): a single-photo post
  or share is just a batch of one, so there is one code path.
- Multi-select via PickMultipleVisualMedia and multi-share via
  ACTION_SEND_MULTIPLE (manifest filter added).
- Per-photo caption and EXIF-prefilled date; each card shows its own
  upload status, with a summary and retry-only-failures on re-post.
- Enqueue one WorkManager worker per photo, reusing the durable retry
  queue. UploadQueue now appends a per-item suffix to the slug so a burst
  enqueued in the same second cannot collide and be dropped by
  enqueueUniqueWork.

android/app/src/main/java/ws/inflo/photogallery/MainActivity.kt

@@ -39,8 +39,8 @@ class MainActivity : ComponentActivity() {
                 var showSettings by rememberSaveable { mutableStateOf(!viewModel.isConfigured) }
 
                 val picker = rememberLauncherForActivityResult(
-                    ActivityResultContracts.PickVisualMedia(),
-                ) { uri -> uri?.let(viewModel::selectImage) }
+                    ActivityResultContracts.PickMultipleVisualMedia(),
+                ) { uris -> if (uris.isNotEmpty()) viewModel.selectImages(uris) }
 
                 val notificationPermission = rememberLauncherForActivityResult(
                     ActivityResultContracts.RequestPermission(),
@@ -73,7 +73,7 @@ class MainActivity : ComponentActivity() {
                 } else {
                     PostScreen(
                         state = viewModel.post,
-                        onPickImage = {
+                        onPickImages = {
                             picker.launch(
                                 PickVisualMediaRequest(
                                     ActivityResultContracts.PickVisualMedia.ImageOnly,
@@ -82,6 +82,7 @@ class MainActivity : ComponentActivity() {
                         },
                         onCaptionChange = viewModel::setCaption,
                         onDateChange = viewModel::setDate,
+                        onRemove = viewModel::removeItem,
                         onSubmit = viewModel::submit,
                         onReset = viewModel::clear,
                         onOpenSettings = { showSettings = true },
@@ -98,16 +99,31 @@ class MainActivity : ComponentActivity() {
         consumeShareIntent(intent)
     }
 
-    /** Picks up a photo handed over by the share sheet. */
+    /** Picks up one or several photos handed over by the share sheet. */
     private fun consumeShareIntent(intent: Intent?) {
-        if (intent?.action != Intent.ACTION_SEND) return
-        val uri: Uri? = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
-            intent.getParcelableExtra(Intent.EXTRA_STREAM, Uri::class.java)
-        } else {
-            @Suppress("DEPRECATION")
-            intent.getParcelableExtra(Intent.EXTRA_STREAM)
+        val uris: List<Uri> = when (intent?.action) {
+            Intent.ACTION_SEND -> {
+                val uri: Uri? = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+                    intent.getParcelableExtra(Intent.EXTRA_STREAM, Uri::class.java)
+                } else {
+                    @Suppress("DEPRECATION")
+                    intent.getParcelableExtra(Intent.EXTRA_STREAM)
+                }
+                listOfNotNull(uri)
+            }
+
+            Intent.ACTION_SEND_MULTIPLE -> {
+                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+                    intent.getParcelableArrayListExtra(Intent.EXTRA_STREAM, Uri::class.java)
+                } else {
+                    @Suppress("DEPRECATION")
+                    intent.getParcelableArrayListExtra<Uri>(Intent.EXTRA_STREAM)
+                }.orEmpty()
+            }
+
+            else -> return
         }
-        uri?.let(viewModel::selectImage)
+        if (uris.isNotEmpty()) viewModel.selectImages(uris)
     }
 
     private fun openUrl(url: String) {