Add GET_CONTENT picker support, fix account/login bugs, restyle selection UI

- Let Noo be picked as a source (Files + Photos) by other apps' native
  file/photo pickers via Android's GET_CONTENT intent, mirroring the
  existing "Share to Noo" plumbing: MainActivity.kt/PickIntentService/
  PickRequest, ServerProvider.pickRequest/confirmPick/cancelPick, and
  pick-mode tap handling in FilesView/PhotosView. While picking, the
  bottom nav and MoreTabsButton are restricted to Files/Photos only.
- Fix two account bugs: testConnection() failures no longer delete a
  still-valid stored app password (only an actual 401 does), and
  switchAccount() now retries a nominally-active-but-logged-out account
  instead of no-op'ing - both were silently locking users out of "Continue
  as" with no way back in except removing and re-adding the account
  (now exposed via login_view.dart's inline remove button).
- Fix the login WebView: clear cookies before every load so "Add Account"
  can't silently reuse an existing session, and push it as a
  non-animated route to avoid a known WebView-as-PlatformView black-screen-
  on-pop issue under Impeller.
- Redesign the Files/Photos selection toolbar to take over the top app bar
  entirely (SyncedHeaderScaffold.selectionBar) instead of sitting as a
  second bar under the controls row, with a horizontally-scrollable,
  uniformly-colored actions row and the same one-shot scroll-hint nudge
  the controls row uses.
- Redesign ShareUploadView to share the same SyncedHeaderScaffold chrome
  as every tab, with the uploading-file summary (marqueed via the new
  shared MarqueeTitle widget) and upload button grouped into one
  rounded/elevated bottom sheet.
- Restyle ShareSheet to open on the same DetailsHeader every other
  per-item sheet uses, and fix a couple of small design-language drifts
  (hardcoded text style, a non-rounded icon).
- Settings' account card: icon buttons with tooltips instead of text
  buttons for refresh/logout/remove, divider removed.

android/app/src/main/AndroidManifest.xml

@@ -49,11 +49,34 @@
                 <category android:name="android.intent.category.DEFAULT"/>
                 <data android:mimeType="*/*"/>
             </intent-filter>
+            <!-- Lets Noo be picked as a source ("Google Photos", "Files", ...)
+                 from another app's own file/photo picker chooser - handled by
+                 MainActivity.kt/PickIntentService, not a plugin. A single
+                 "*/*" filter matches any mimeType the caller requests
+                 (image/*, video/*, or a generic file pick). -->
+            <intent-filter>
+                <action android:name="android.intent.action.GET_CONTENT"/>
+                <category android:name="android.intent.category.DEFAULT"/>
+                <category android:name="android.intent.category.OPENABLE"/>
+                <data android:mimeType="*/*"/>
+            </intent-filter>
         </activity>
         <service
             android:name=".ShareUploadService"
             android:exported="false"
             android:foregroundServiceType="dataSync"/>
+        <!-- Hands back the file(s) a caller picked via the GET_CONTENT filter
+             above - only the app's own cache/picker/ subfolder is exposed,
+             see file_paths.xml. -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.picker.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths"/>
+        </provider>
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data