Android: write and read picked files through content URIs

On Android the file dialogs return content:// URIs, not paths. Exports
wrote them with std::fs, which failed after the system had already
created the document, leaving a 0 B file; deck, library-card and library
backup exports now write through tauri-plugin-fs, and a failed deck
export shows an error instead of failing silently.

Library-file and card-database imports read the pick through the fs
plugin too. A library file is told apart by its SQLite header rather
than its extension, and SQLite opens a local copy. The library-import
picker drops its filters on Android, where they hid .db and backup files.

Deck import suggests the file's display name rather than the URI's
opaque id, and the backup's "Saved to" message shows the file name.

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

src-tauri/gen/android/app/src/main/java/com/garrettmills/mtgc/SharePlugin.kt

@@ -3,11 +3,14 @@ package com.garrettmills.mtgc
 import android.app.Activity
 import android.content.ClipData
 import android.content.Intent
+import android.net.Uri
+import android.provider.OpenableColumns
 import androidx.core.content.FileProvider
 import app.tauri.annotation.Command
 import app.tauri.annotation.InvokeArg
 import app.tauri.annotation.TauriPlugin
 import app.tauri.plugin.Invoke
+import app.tauri.plugin.JSObject
 import app.tauri.plugin.Plugin
 import java.io.File
 
@@ -18,9 +21,16 @@ class ShareImageArgs {
   var title: String? = null
 }
 
+@InvokeArg
+class DisplayNameArgs {
+  /** A content:// URI the system file picker returned. */
+  lateinit var uri: String
+}
+
 /**
  * The Android half of the app's `share` plugin (src-tauri/src/share.rs):
- * opens the system share sheet for a card image the Rust side has saved.
+ * opens the system share sheet for a card image the Rust side has saved, and
+ * looks up the names of documents the system file picker returns.
  */
 @TauriPlugin
 class SharePlugin(private val activity: Activity) : Plugin(activity) {
@@ -48,4 +58,18 @@ class SharePlugin(private val activity: Activity) : Plugin(activity) {
       invoke.reject(ex.message)
     }
   }
+
+  /** The file name the picker showed for `uri`, e.g. "Atraxa.txt". */
+  @Command
+  fun displayName(invoke: Invoke) {
+    try {
+      val args = invoke.parseArgs(DisplayNameArgs::class.java)
+      val name = activity.contentResolver
+        .query(Uri.parse(args.uri), arrayOf(OpenableColumns.DISPLAY_NAME), null, null, null)
+        ?.use { if (it.moveToFirst()) it.getString(0) else null }
+      invoke.resolve(JSObject().apply { put("name", name) })
+    } catch (ex: Exception) {
+      invoke.reject(ex.message)
+    }
+  }
 }