fix(thread): show every message of the thread, not just the tapped one

[lucletoffe]

Problem

The email list collapses same-thread messages into one row with a count badge — but tapping that row shows a single message. EmailThreadScreen receives a threadId route param and never uses it: each pager pane renders only the tapped email, and Prev/Next walk the flat mailbox list. The rest of the conversation — including your own replies filed in Sent — is unreachable from the thread view.

What this PR does

Each pager pane now renders the whole thread, oldest first:

• The thread is fetched once per threadId (Thread/get + a light Email/get, coalesced and cached across the three mounted panes) and follows the server's ordering (RFC 8621 §3).
• Messages other than the tapped one start collapsed: sender, one preview line, date, unread dot. Tapping unfolds a message in place, loads its full body on demand, and marks it read. Tapping an expanded message's header folds it back.
• The tapped message is expanded from the start and keeps the existing configurable mark-as-read delay.
• The subject row shows the real thread size — which can exceed the list badge, since that badge only counts messages in the open folder.
• A thread fetch failure (or a single-message thread) degrades to the exact previous single-message view.

Implementation notes

• The former pane body (sender header, attachment chips, calendar banner, EmailBodyView) is extracted as MessageSection, rendered once per expanded message; the single-message path renders it identically to before (header stays non-pressable there).
• Mark-read on expand: a thread member living in another folder isn't in the store's list, and the store's markRead would rebuild its keyword map from nothing — JMAP Email/set replaces keywords wholesale, wiping $flagged and user labels server-side. Such messages go through setEmailKeywords with the keywords merged from the fetched copy instead; in-list messages keep the store's optimistic path.
• Shared/group accounts: Thread/get/Email/get ride the pane's ownerAccountId.
• orderThreadEmails (server order, drops ids whose metadata didn't come back) is a pure function in lib/email-threading.ts, unit-tested.

Tests

• orderThreadEmails: server-order preservation, expunged-id drop, empty thread.
• npx tsc --noEmit clean; vitest 438/438 passing (the auth-store.test.ts collection error is pre-existing on main).

🤖 Generated with Claude Code

https://claude.ai/code/session_01ACzTUX8qVZ4qAp8gqUHb5t