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-preload.ts

@@ -0,0 +1,7 @@
+import { contextBridge, ipcRenderer } from 'electron'
+
+contextBridge.exposeInMainWorld('streamingInputBridge', {
+  onChunk: (callback: (delta: string) => void) => {
+    ipcRenderer.on('streaming-input:chunk', (_event, delta) => callback(delta))
+  }
+})