feat: add multi-select share and batch convert in downloaded/local album screens

- Add shareMultipleContentUris native handler in MainActivity for ACTION_SEND_MULTIPLE
- Add shareMultipleContentUris binding in PlatformBridge
- Add _shareSelected and _performBatchConversion methods to DownloadedAlbumScreen and LocalAlbumScreen
- Add batch convert bottom sheet UI with format/bitrate selection (MP3/Opus, 128k-320k)
- Add share & convert action buttons to selection bottom bar in both screens
- Add batch convert with full SAF support: temp copy, write-back, history update
- Add share/convert selection strings to l10n (all supported locales + app_en.arb)
- Add queue tab selection share/convert feature (queue_tab.dart)
- Update donate page
- Update go.sum with bumped dependency hashes

.gitattributes

@@ -0,0 +1,20 @@
+* text=auto eol=lf
+
+# Windows scripts
+*.bat text eol=crlf
+*.cmd text eol=crlf
+*.ps1 text eol=crlf
+
+# Binary files
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.webp binary
+*.ico binary
+*.pdf binary
+*.zip binary
+*.jar binary
+*.aar binary
+*.keystore binary
+*.jks binary