Add multi-account: unified inbox with per-account filter

The data layer, background sync, and IDLE already handled N accounts; this makes
the UI account-aware.

- Mailbox: filter chips (All + one per account) appear once 2+ accounts exist, and
  each message in the unified view is labelled with its account. The filter resets to
  All if the selected account is removed.
- Reply now carries the receiving account through to compose, so From defaults to the
  account that received the message rather than just the first account.
- Removing an account now also deletes its cached messages and attachments, so they
  leave the unified inbox.
- assemble/test/lint green; verified on the Android 17 emulator — added a second
  GreenMail account, saw both accounts' mail unified + attributed, filtered to one
  account, and replied from the correct account.

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

README.md

@@ -13,8 +13,9 @@ experience with power-user features tucked under an **Advanced Settings** group.
 > with mark-read, star, and delete; and **composing** — a compose screen with device-
 > contacts autocomplete that sends over SMTP, plus reply; **on-device new-mail
 > notifications** (no push service) with persisted settings; **instant push** via a
-> foreground **IMAP IDLE** service; and **attachments** — downloaded on demand and opened
-> in a system viewer. Outlook Graph send and multi-account polish are upcoming.
+> foreground **IMAP IDLE** service; **attachments** — downloaded on demand and opened in a
+> system viewer; and **multiple accounts** — a unified inbox with per-account filtering.
+> Outlook Graph send and search are upcoming.
 
 ## Features (target MVP)