feat(ios): add the GRDB offline mail cache

[Tutitoos]

Plan reference

Post-1.0 — iPhone and iPad: local cache.

Objective

Provide an encrypted, account-partitioned GRDB cache for a 90-day mail window, offline reads, pending actions, drafts, migrations, eviction, and safe rebuild.

Scope

• Model cached accounts, folders, threads, messages, body variants, attachment metadata, draft revisions, action queue, event cursor, and sync metadata.
• Add migrations, observation, eviction, account erasure, corruption recovery, and reconnect reconciliation.

Tasks

• Define Swift 6 models, actors, storage, API, and UI boundaries for this scope.
• Implement the smallest complete native capability.
• Add XCTest, Swift Concurrency, migration, UI, accessibility, and device tests where applicable.
• Update generated clients, translations, privacy declarations, and distribution documentation.

Acceptance criteria

• Cached inbox, search subset, and opened conversations remain usable offline.

• Removing an account erases its rows and keys.

• Corruption or migration failure offers safe rebuild without remote mutation.

• Reconnect reconciles server events, local drafts, and pending actions deterministically.

• Account isolation, data protection, accessibility, EN default, and ES completeness match the web and macOS contracts.

• Tokens, credentials, message content, recipients, and personal data never enter logs, crash reports, screenshots, fixtures, URLs, or notification defaults.

• The capability is independently reviewable and rollback-safe.

Validation

• Run GRDB migration, encryption, observation, offline, reconnect, conflict, eviction, corruption, and large-cache performance tests.

• Run Swift build, XCTest, strict concurrency checks, generated-client diff, and the affected simulator and physical-device UI tests.

• Use sanitized fixtures and protected accounts only for final manual provider validation.

Dependencies

Post-1.0 work. Blocked until Mailflow 1.0.0 is released: https://github.com/Tutitoos/mailflow/milestone/9

Risks

Offline state, background execution, notification privacy, generated contracts, and device credentials can diverge from server truth. Use actors, Keychain, bounded caches, explicit conflict states, and real-device validation.

Out of scope

• Full historical archive, provider authority, and shared database with macOS.