test(crypto): add recovery + rotation coverage (rotation had zero tests)

[h4yfans]

Closes Phase 1 §1.6 of .claude/plans/tech-debt-remediation.md.

Summary

Dedicated coverage for recovery.ts (55L) + rotation.ts (339L). Rotation had zero tests prior to this PR — highest-risk unit in Phase 1.

New files

• recovery.test.ts (207L) — 14 tests across 5 exports. Real bip39 wordlist + real libsodium. Covers generateRecoveryPhrase (24-word BIP-39, valid checksum, 64-byte seed, uniqueness), validateRecoveryPhrase (valid/bad-checksum/non-wordlist/empty), phraseToSeed determinism, recoverMasterKeyFromPhrase roundtrip vs deriveMasterKey, validateKeyVerifier.

• rotation.test.ts (779L) — 13 tests across all rotation exports. Real crypto end-to-end.

  • rewrapItemKey — file-key recovery via new vault key; encryptedData/dataNonce/signerDeviceId/originalId/clock/stateVector/deletedAt survive; new signature verifies; malformed key-nonce throws.
  • rewrapCrdtSnapshot — Yjs body byte-identical pre/post, wrapped-key region differs, file key recovered matches, new signature verifies.
  • performKeyRotation — happy path (phases: preparing → re-encrypting → finalizing → complete), processedItems monotonic to totalItems, pause+resume once, pushed items signed under new vault key, server keys + master-key store invoked.
  • Early-return: null token / null signing keys / null vault key.
  • Mid-rotation failure: malformed keyNonce on item index 1 → pushItems excludes failing id, error matches "Rotation aborted: 1 items failed re-wrap", server keys NOT advanced, sync resumed.
  • Server-rejected push item; CRDT snapshot rewrap failure; unexpected dep throw still resumes sync.

Note on mid-rotation semantics

The spec envisioned "items 0..N-1 kept new key, N..end kept old key". The actual rotation.ts:238-269,301-305 catches per-item rewrap errors and aborts at the post-loop guard with Rotation aborted: N items failed re-wrap. This PR tests the real behavior; the safety post-conditions that matter (server keys NOT updated, master key NOT stored, sync resumed) are asserted.

Test plan

• pnpm --filter desktop test --project main src/main/crypto/recovery.test.ts src/main/crypto/rotation.test.ts — 0 failures
• per-file coverage on both ≥ 95%
• pnpm typecheck:node && pnpm typecheck:web clean
• pnpm lint clean

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