fix(ui): keep unsent composer drafts per conversation

[CatJuly]

Summary

With multiple task sessions open, typing into one session's composer and then switching to another task discards the draft — in both sessions. Whatever the user had typed is simply gone.

Repro: open two task sessions → type into session A's composer without sending → click session B in the sidebar → switch back to A. The composer is empty in both A and B.

Root cause

Two independent paths throw the text away on a session switch:

1. ContextPanel mounts the composer with key={channelId}, so switching sessions unmounts and recreates the component — and text lives in plain useState('').
2. MessageComposer additionally has a channelId effect that explicitly resets the text to '' on every change, so even a non-remounting switch wipes it.

Neither path saves the draft anywhere first.

Fix

Follow the convention chat products use: a draft belongs to its conversation.

• Keep unsent drafts in a module-level Map<channelId, string> (survives the key-driven remounts).
• Every keystroke writes through to the map.
• Mounting or switching restores the current channel's draft instead of wiping it.
• A successful send clears that channel's entry.

Attachments intentionally still reset on switch: their object URLs and upload state are tied to the current mount, and preserving them would leak resources.

Testing

• npm run typecheck — clean
• npm run build — clean; rebuilt frontend_dist bundle included
• Manual: typed in session A → switched to B (composer empty, no cross-talk) → typed in B → switched back and forth: both drafts restored to their own sessions; sending in A cleared only A's draft; drafts survive repeated switches

Scope

Frontend only (MessageComposer.tsx + rebuilt dist). Independent of my other open PRs — no shared files.