Messages: honor GET /api/limits and handle subscriber/verification gates #37

Summary

The server publishes its own media and content limits; the app hard-codes nothing and validates nothing, so an oversized upload fails server-side with a generic error.

Live verification (2026-09-15)

GET /api/limits -> 200
{"media":{"image":{"maxBytes":1468006,"maxPixels":1200,"acceptedFormats":["jpeg","png","gif","webp"]},
          "video":{"maxBytes":3145728,"acceptedFormats":["mp4","mov"]}},
 "message":{"maxContentLength":5000}}

Gates to handle as first-class states, not generic errors

From /help/account:

• Email must be verified before posting messages or attaching media (emailVerified on GET /api/user).
• Account status new (on probation) locks DMs, image/video upload, cross-posting, scheduled posts, and creating lists/documents/organizations, with a few posts per hour.
• Media, cross-posting and scheduling are Subscriber features (customerStatus).
• Effective message length is the caller's own maxMessageLength (GET /api/user), which may be below the server cap.

Acceptance criteria

• GetLimitsAsync added and cached for the session.
• The file picker filters to acceptedFormats; oversized or over-dimension files are rejected client-side with the actual limit quoted.
• Composer counter uses min(user.maxMessageLength, limits.message.maxContentLength).
• 402/403 on a gated action renders an upgrade-needed or verify-email state naming the reason.
• Media/cross-post/schedule controls are disabled with an explanation when the account status or tier forbids them.