perf: narrow streaming animation redraw to timeline list only

[TigerInYourDream]

Summary

• Replace self.redraw(cx) with redraw_timeline_list() that only redraws the timeline PortalList, skipping input bar and typing notice
• Skip redraw entirely when streaming messages are not visible in viewport
• Apply same optimization to streaming timeout handler

Closes #53

Context

During streaming message animation (55ms tick, 2-char reveal), self.redraw(cx) was called on the entire RoomScreen subtree. This unnecessarily redraws static UI elements (input bar, typing notice) and triggers redraw even when streaming messages have scrolled off-screen.

Changes

• New redraw_timeline_list() method that targets only timeline.list PortalList
• New any_timeline_indices_visible() helper to check if any streaming item is in the viewport before triggering redraw
• Both animation frame handler and timeout handler now use the narrowed scope

Test plan

• Streaming animation still works correctly (character-by-character reveal)
• No visual glitches when scrolling during active streaming
• Verify reduced redraw scope via debug log (Streaming animation frame took Xms)