perf(chat): make session switching feel instant

Switching sessions ran as one synchronous commit: sidebar highlight, URL,
a full timeline remount with markdown re-parse, and around nine requests,
so nothing changed on screen for 150-250ms after the click.

- ChatContainer swaps the timeline on a deferred copy of the selection, so
  the active row, URL, and tab commit first and the timeline renders behind
  them; selection policy keeps reading the live store value.
- The message fetch starts before the selection is published.
- Sidebar rows stop re-rendering on a project switch: directory-scoped sync
  hooks read the runtime context and a subscribable current-directory source
  instead of the directory-bearing context; the grouping builder reads git
  branches through a ref and section caches key the branches they use;
  descendant ids are keyed by content. Rows per switch went from 73 to 8.
- Markdown skips the async re-render when the settled cached blocks are
  already painted, and mounts synchronously once its lazy module is loaded;
  the module is preloaded at boot.
- A timeline reveal gate holds a freshly opened session at opacity 0 while
  any provisional markdown paint catches up (250ms cap), then fades the whole
  timeline in once, so text, tools, and recap appear together.
- Switch fan-out trimmed: knowledge summary deduped, MCP status refreshed only
  when stale, non-repo directories cached by the git repo check, OpenChamber
  defaults cached briefly, agent memory reused for the same project, goal
  text cached, PWA manifest rebuilt after the switch settles.
- Header tabs snap into the active state and keep the title at the same
  height in both states.
- Prefetch on row press; composer focus moved off the commit.

`bun run profile:switch` records ack/content latency, longest task, and
requests per switch, cold and warm, and compares runs against a baseline.
Measured warm switch: ack 228ms to about 40-60ms, content 228ms to about
100-120ms.

packages/ui/src/stores/useConfigStore.ts

@@ -67,7 +67,26 @@ interface OpenChamberDefaults {
     sttLanguage?: string;
 }
 
-const fetchOpenChamberDefaults = async (): Promise<OpenChamberDefaults> => {
+// Directory activation re-reads the OpenChamber defaults, which are global,
+// not per directory: one request serves the switches that land inside this
+// window, and concurrent activations share the in-flight one.
+const OPENCHAMBER_DEFAULTS_FRESH_MS = 15_000;
+let openChamberDefaultsCache: { at: number; request: Promise<OpenChamberDefaults> } | null = null;
+
+const fetchOpenChamberDefaults = (): Promise<OpenChamberDefaults> => {
+    const now = Date.now();
+    if (openChamberDefaultsCache && now - openChamberDefaultsCache.at < OPENCHAMBER_DEFAULTS_FRESH_MS) {
+        return openChamberDefaultsCache.request;
+    }
+    const request = requestOpenChamberDefaults();
+    openChamberDefaultsCache = { at: now, request };
+    request.catch(() => {
+        if (openChamberDefaultsCache?.request === request) openChamberDefaultsCache = null;
+    });
+    return request;
+};
+
+const requestOpenChamberDefaults = async (): Promise<OpenChamberDefaults> => {
     markStartupTrace('config.defaults:start');
     const started = typeof performance !== 'undefined' ? performance.now() : Date.now();
     const finish = (source: string, result: OpenChamberDefaults) => {