Composer: honour the account's character limit and the tightest cross-post limit #24

Part of #17 — Message composer & feed parity gaps

Each account has a configurable message length limit (default 666 characters), set in
Settings → Profile and exposed as maxMessageLength on PATCH /api/user/update.

Additionally, when cross-post targets are selected the binding limit is the tightest one among
them — Bluesky's 300 is usually the constraint — not InterlinedList's own.

• Read the account limit and enforce it in the composer with a live counter.
• Recompute the effective limit as destinations are toggled, and show which platform is binding.
• Over-limit drafts should offer the thread path rather than a hard block, matching the web.

Tests: counter reflects the account limit; toggling Bluesky tightens it to 300.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.