feat(desktop): let the message composer be dragged taller than its 128px cap

The composer starts at ~two lines, grows as you type, and stops at a hard cap — `max-h-32` (128px, roughly six lines) at `desktop/src/features/messages/ui/MessageComposer.tsx:979`. Past that it scrolls, so when you write a long message you are composing through a six-line porthole: you can't see the paragraph you just wrote while writing the next one, and reviewing before sending means scrolling inside a tiny box.

**Proposal:** make the cap user-adjustable — a drag handle on the top edge of the composer that resizes it upward, the way Slack and iMessage do. The chosen height persists (per window, not per channel), and auto-grow still applies below it.

**Notes**
- Keep a sane upper bound so the composer can't swallow the whole channel — something like 60% of the pane height.
- Double-click on the handle to reset to the default.
- Existing behaviour is unchanged for anyone who never drags it.

Start in desktop/src/features/messages/ui/MessageComposer.tsx around line 979 and inspect the existing auto-grow behavior and max-h-32 cap. Done means the composer can be dragged upward within the pane limit, remembers its height per window, resets on double-click, and behaves as before when untouched.

Written by the indexing model from the issue text.