perf: reduce streaming render cost and refresh activity page on demand

- Segment streaming Markdown so completed blocks are reused and only the growing tail is re-parsed; fall back to whole-document parsing for document-wide constructs
- Memoize conversation sidebar rows and cache per-conversation derived activity, ordering and pending approvals
- Activity page loads on entry, refreshes every 15 s while visible, and has a single header refresh button
- Include pending settings, model download source, device sharing and knowledge gateway changes in the working tree
- Add Electron streaming input benchmark and regression tests

tests/support/streaming-input-page.html

@@ -0,0 +1,3 @@
+<!doctype html>
+<html lang="en"><head><meta charset="utf-8"><title>Streaming native input benchmark</title></head>
+<body><div id="root"></div><script type="module" src="./streaming-input-renderer.tsx"></script></body></html>