Add share-to-upload, per-folder sort, and media viewer polish

- Accept Android's "Share to..." intent (ACTION_SEND/SEND_MULTIPLE) to
  upload a shared file, with a new destination-folder picker screen
- Unlink Files/Photos sort settings and make Files remember sort
  per folder (like per-folder view settings), instead of one shared
  global sort field for both tabs
- Media viewer: show the file name (marquee-scrolling if too long) in
  the top bar next to a resized back button, slide the top bar out of
  view instead of fading it, and unify icon color/contrast across the
  top bar, bottom action bar, and video transport controls
- Add a scrollbar thumb to every scrollable list in the app (all tab
  views via SyncedHeaderScaffold, search results, settings, the new
  upload folder picker, share sheet, and the Details sheet's tabs)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -10,7 +10,7 @@
         <activity
             android:name=".MainActivity"
             android:exported="true"
-            android:launchMode="singleTop"
+            android:launchMode="singleTask"
             android:taskAffinity=""
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
@@ -28,6 +28,18 @@
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
+            <!-- Lets other apps' share sheets ("Share to...") send a file here
+                 to be uploaded, via receive_sharing_intent. -->
+            <intent-filter>
+                <action android:name="android.intent.action.SEND"/>
+                <category android:name="android.intent.category.DEFAULT"/>
+                <data android:mimeType="*/*"/>
+            </intent-filter>
+            <intent-filter>
+                <action android:name="android.intent.action.SEND_MULTIPLE"/>
+                <category android:name="android.intent.category.DEFAULT"/>
+                <data android:mimeType="*/*"/>
+            </intent-filter>
         </activity>
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->