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

src/shared/goodbuddy-config-contracts.test.ts

@@ -159,6 +159,7 @@ describe('GoodBuddy configuration contracts', () => {
         localToolEnvironment: defaultLocalToolEnvironmentSettings,
         applicationNavigation: defaultApplicationNavigation,
         localInferenceEnabled: true,
+        deviceSharingEnabled: false,
         conversationHtmlRenderingEnabled: true,
         remoteProjectsEnabled: false,
         magicNotesEnabled: true,