feat: incremental (delta) sync for mailbox refresh

[Aravinda-HWK]

What

Implements Phase 3 (delta sync) of the Email 2.0 proposal: refreshing a mailbox now fetches only what changed instead of re-listing the whole folder.

Because go-imap v1 exposes no CONDSTORE/MODSEQ, the delta is computed from UIDs + flags rather than a MODSEQ token (the proposal's documented fallback in §10).

How it works

The client sends the UIDs it already holds + its cached UIDVALIDITY; the server returns only:

• added — full envelopes for UIDs newer than the client's highest known UID (UID SEARCH (since+1):*)
• flags — current flags for known UIDs still present (client diffs locally to update read/unread)
• removed — known UIDs that have been expunged/moved
• resync — set when UIDVALIDITY changed, telling the client to discard its cache and refetch

Changes

Server

• New GET /api/v1/mailboxes/{mailbox}/changes → MailboxDelta
• imap.MailboxChanges + shared fetchEnvelopes/fetchFlags helpers
• ListMessages now also returns uidvalidity (baseline for the client)

Client

• Dexie v2 syncMeta table persists uidvalidity per folder
• mailboxes.changes() endpoint + MailboxDelta/FlagUpdate types
• DataContext reconciles added/removed/flag changes idempotently (keyed on UID); new refreshFolder(role) and a ThreadList refresh button wired into Inbox/Sent/Trash

Performance

• refreshFolder syncs only the viewed folder, so four folder syncs no longer serialize on the session's single IMAP connection
• IMAPFor drops a redundant per-request NOOP probe (one RTT) — every operation already self-heals via ensureLive

Together these cut a refresh from ~4.25s (four folders contending on one connection) to a single sub-second changes request.

Tests

• parseUIDList unit tests added
• go build ./..., go test ./..., and frontend tsc --noEmit all pass

Notes / follow-ups

• This is the UID+flag-diff approach (no library change). True MODSEQ-based sync would need a go-imap/v2 migration (currently beta).
• Natural next steps from the proposal: connection pool (§7) for true parallel folder syncs, and Phase 4 (IMAP IDLE → WebSocket push).