fix(chat): preserve per-session message drafts

[chliny]

Summary

• Store unsent composer text by session ID.
• Restore the correct draft when navigating between sessions.
• Clear a draft after sending and remove it when its session is deleted.

Drafts remain in memory only and are isolated from the ZeroTier/session-list work.

Verification

• npm test: 333 passed, 0 failed.
• npm run typecheck: passed.
• Confirmed the isolated branch contains no session-list cache or loadSeq changes.
• Android release build: assembleRelease -PreactNativeArchitectures=arm64-v8a passed.