Read-only JMAP Support

Summary

Implement read-only JMAP support: connect to JMAP servers, sync mailbox structure, and fetch messages.

Problem

Users with JMAP-enabled accounts cannot access their mail via Nextcloud Mail.
We need to add JMAP as a supported protocol with feature parity to IMAP for reading mail.

Solution

• Create JMAP client factory using SebastianKrupinski/jmap-client-php
• Implement mailbox structure sync and conversion to domain models
• Implement message fetching and conversion to domain models
• Handle JMAP Session endpoint discovery and state-based sync
• Ensure output uses same domain models as IMAP

Implementation

Create lib/JMAP/ namespace for JMAP-specific logic:

• JmapClientFactory - creates and configures JMAP client
• JmapMailboxMapper - converts JMAP Mailbox objects to domain models
• JmapMailboxSync - syncs folder structure into database
• JmapMessageMapper - converts JMAP Message objects to MessageDTO
• JmapSync - orchestrates message sync using JMAP state tokens

Credential handling reuses existing encryption patterns.

Session discovery fetches .well-known/jmap for JMAP capabilities.

Sync strategy:

• Full sync on first run
• Incremental sync using JMAP state tokens on subsequent runs
• Detects new/changed/deleted messages

Message extraction:

• Headers (To, From, Subject, Date, Message-ID, References, In-Reply-To)
• Flags and keywords
• Attachment metadata
• Message IDs for threading

Affected Components

• New: lib/JMAP/ directory with mappers and sync logic
• ProtocolClientFactory: register JmapClientFactory (from https://github.com/nextcloud/mail/issues/12609)
• SyncService: use dispatcher to call appropriate sync
• composer.json: add SebastianKrupinski/jmap-client-php dependency

Acceptance Criteria

• JMAP accounts can be created (via https://github.com/nextcloud/mail/issues/12609 infrastructure)
• Connection test validates JMAP server availability
• Mailbox structure syncs correctly
• Messages fetch with all required properties
• Output matches IMAP mapper format (same domain models)
• Existing IMAP sync unaffected

Out of Scope

• Message mutations (flags, move, delete) - see https://github.com/nextcloud/mail/issues/12611
• UI changes - see https://github.com/nextcloud/mail/issues/12612
• Auto-detection

Start by reading the existing IMAP sync and mapper flow, then inspect ProtocolClientFactory, SyncService, composer.json, and the proposed lib/JMAP/ namespace. Confirm how the JMAP client dependency, session discovery, state-based synchronization, and domain model conversions fit the existing flow. Done means read-only JMAP accounts sync mailboxes and messages with the same models while IMAP behavior remains unchanged.