fix: skip raw on open, validate folders, decode cached subjects, map move conflicts

[TheGreatAxios]

Summary

• Stacked on fix(routes): harden POST /me/inbox/send #54.
• openNativeMailboxStore no longer selects raw. readRaw fetches it when needed, so a list or mark-read no longer loads every frame in the folder.
• POST /me/inbox/:uid/read and /unread return 400 for a ?folder= outside INBOX, Sent, Archive and Trash, so an unknown folder never creates a mailbox_state row. restore returns 400 for any source except Archive or Trash (the README says so).
• persist decodes RFC 2047 encoded-words (B and Q) in the Subject it caches. @intx/mime has no decoder, so decodeEncodedWords lives in frame.ts. A word that does not decode (invalid base64, bytes invalid in its charset, an unknown charset) is left as written, and whitespace is dropped only between two encoded-words that both decode, never after a literal ?= or before an undecodable word.
• moveNativeMailboxMessage runs in one transaction, so a failed move burns no uid. It locks both folders' mailbox_state rows up front in folder order, so opposite-direction moves queue instead of deadlocking. The archive/trash/restore routes return 404 only when the uid is missing, 409 when the destination already holds the Message-ID from the same sender (Postgres 23505), and 500 for anything else, such as a database outage.

Verification

• e2e/mount-native.test.ts: restore?folder=INBOX|Sent|junk is 400; read/unread?folder=junk is 400 with no mailbox_state row; archiving onto a Message-ID Archive already holds is 409 with both folders' uid_next unchanged; a move against a closed connection is 500. 8 archive and 8 restore moves run interleaved return only 200/404/409 and leave 16 distinct rows with distinct (folder, uid); on fix(routes): harden POST /me/inbox/send #54 it gets deadlock 500s.
• e2e/persist.test.ts: =?UTF-8?B?w6nDqcOp?= is cached as ééé.
• src/frame.test.ts unit-tests decodeEncodedWords, including =?UTF-8?B?!!!!?= and a truncated UTF-8 sequence staying as written, and plain?= =?UTF-8?Q?b?= keeping its space, and =?UTF-8?Q?a?= =?UTF-8?B?!!!!?= keeping the space before the undecodable word.
• These tests fail on fix(routes): harden POST /me/inbox/send #54. Nothing tests the dropped raw column directly; the readRaw tests cover it.
• bun run check, build, test:e2e and the pack smoke pass locally.

Closes CL-9437