android: shows and attaches files, and the share sheet takes images

The phone downloaded every attachment and drew none of them, so a photo note
looked empty. Now:

- Cards show a note's first image and name its other files; the editor shows
  every image at full width and every file as a row. Tapping one opens it in
  whatever app handles its type (a cache copy under its real name, through a
  FileProvider that serves only those copies). Each can be removed, and a file
  the server refused says why under it.
- The editor's toolbar has an Attach button (any type, several at once). Files
  are stored on the phone straight away and upload on the next sync that
  reaches a server, through the core's step-6 path. Link previews show in the
  editor too, and can be dismissed.
- Share → Inkwell accepts one or several images, with or without a caption,
  finishing #1899's deferred image/* target.
- The FFI gains add_attachment, delete_attachment, delete_preview and
  blob_path. The sync summary counts uploads and failed uploads.

Images decode at the size they are drawn (BitmapFactory sampling plus EXIF
rotation, small LRU cache), so no image library is added. Files over 50 MB are
refused on the phone before they are read whole into memory.

Task #5169.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -125,17 +125,27 @@
               Capture without opening the app first: Share → Inkwell from
               anywhere, and the selection toolbar in any text field.
 
-              text/plain ONLY, and image/* deliberately absent. Nothing in this
-              app can create an attachment — the core has `delete_attachment` and
-              no counterpart, and the FFI exposes neither. Claiming images in the
-              share sheet would put this app in front of people for a job it
-              cannot do and fail after they had chosen it.
+              Text, and images one at a time or several at once. A shared image
+              becomes a note carrying it as an attachment, stored on the phone
+              and uploaded on the next sync that reaches a server (#5169).
+
+              image/* rather than */*: a photo or a screenshot is what people
+              share into a notes app. Claiming every type would put Inkwell in the
+              share sheet for APKs, contacts and calendar entries, where it would
+              be noise. Other files attach from inside the editor, whose picker
+              takes any type.
             -->
             <intent-filter>
                 <action android:name="android.intent.action.SEND" />
                 <category android:name="android.intent.category.DEFAULT" />
                 <data android:mimeType="text/plain" />
             </intent-filter>
+            <intent-filter>
+                <action android:name="android.intent.action.SEND" />
+                <action android:name="android.intent.action.SEND_MULTIPLE" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <data android:mimeType="image/*" />
+            </intent-filter>
 
             <!--
               The label is what appears in the text-selection menu beside Copy and
@@ -164,6 +174,22 @@
           PackageInstaller. Without it a failed install would be indistinguishable
           from someone declining the dialog (Scribe #2438).
         -->
+        <!--
+          Hands an attachment to the app that opens its type. Not exported, as a
+          FileProvider must not be: access is granted one URI at a time, on the
+          intent that opens it. It serves only the cache copies listed in
+          res/xml/file_paths.xml, never the note store.
+        -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.files"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
+
         <receiver
             android:name=".UpdateReceiver"
             android:exported="false" />