Summary

Typing in the dashboard composer feels laggy on long sessions (#721).
Every keystroke commits to input state, so ChatPanel re-renders and
Preact walks its entire vnode tree on each frame — the message list,
the side rail, and the status bar.

ChatMessage was already memoised in #560 so historical bubbles
short-circuit individually, but Preact still has to .map() over every
message, evaluate the per-row Boolean(streaming && streaming.id === m.id)
discriminator, and diff each element. On a session with 100+ messages,
the per-keystroke work is enough to drop frames.

Fix

• Lift the message list into a <ChatFeed> sub-component and wrap it
  in memo. While the user types, messages and streaming refs are
  stable, so memo bails the whole feed out — keystrokes no longer
  touch the feed.
• Wrap SideRail and ChatStatusBar in memo for the same reason —
  /overview poll churn and input edits won't repaint progress bars or
  the statusbar between actual state changes.

Closes #721

Test plan

• Open dashboard with a long-running session (50+ messages), type
  in the composer, confirm typing is responsive.
• Verify auto-scroll on new message still works (feedRef passed
  through innerRef).
• Confirm streaming bubble still updates in real time (memo
  re-renders when streaming ref changes per rAF flush).
• /overview poll keeps refreshing status bar / side rail values
  every 2.5s.
• Language switch still updates feed empty-state text.