feat: long-press multi-select, aggressive prefetch, and download indicators

Add a long-press contextual action bar to the mailbox: Archive, Delete,
Spam, Move, Select All, and (single-selection only) Reply, Reply All, and
Forward. Reply All/Spam/Delete are confirmed first; deleting from Trash or
Spam warns that it is permanent.

- Delete moves to Trash and Spam moves to the Spam folder; only deletes
  already in Trash/Spam do a permanent IMAP expunge. Archive/Spam/Move
  resolve the destination per account so the unified inbox works.
- Reply/Reply All/Forward force a fresh server fetch of the original
  (recipients + body via BODY.PEEK), build a quoted draft, and open compose;
  a spinner shows during the fetch and they error via snackbar if offline.

Add a top-level "Message downloading" setting (Always fetch all / Fetch all
on Wi-Fi / Always on-demand; default Always) that aggressively pre-caches
full bodies and all attachment bytes during sync (outside the sync lock,
without marking mail read), into a persistent per-part attachment cache so
messages and attachments open instantly and offline.

Show an "available offline" mark on cached list rows and a per-attachment
"downloaded" check in the reader.

Extract a Syncer interface (MailSyncer implements it) so the mailbox can be
driven end-to-end in tests.

Tests: 66 unit + 27 instrumented, all passing on the API 37 emulator.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -4,6 +4,7 @@
     xmlns:tools="http://schemas.android.com/tools">
 
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.READ_CONTACTS" />
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />