Email/get bacting

[chibenwa]

CF #4261

Implements batched Email/get requests to respect server maxObjectsInGet limits.

Changes

• Added getEmailsByIdsBatched method in MailApiMixin for batched fetching
• Added getMaxObjectsInGetMethod to query server capabilities
• Refactored ThreadApi.getChanges to use batched requests with early termination
• Added defaultMaxObjectsInGet = 50 constant

Why This Is Needed

Stalwart and other JMAP servers enforce maxObjectsInGet limits. When loading threads with many emails (e.g., after WebSocket push notification), the client must batch requests to avoid exceeding server limits.

[hoangdat]

#4261 (comment)