perf: isolate chat streaming renders and reduce sidebar render cost (#1672)

Reworks the chat and session-sidebar render paths to cut render cascades, memory
  churn, and UI jank on large sessions and big session trees. Behavior is preserved;
  the changes are about *when* and *how much* the UI re-renders.

  ## Chat streaming
  - Freeze the streaming message's parts in the bulk turn projection during streaming,
    and re-inject live parts only in an isolated tail leaf, so a ~60/sec delta stream
    no longer re-runs the whole-session projection or re-renders unrelated rows.
    session with referential reuse of unchanged turns.
  - Memoize message rows with field-aware comparators instead of reference equality.
  - Replace the manual child-session polling in the task tool with the live SSE
    stream + a one-shot load, removing a fetch/settle state machine.

  ## History loading & scroll
  - Load an initial page fast, then prepend one older page in the background so the
    scroll container has headroom and "load older on scroll-up" fires before the user
    hits the absolute top.
  - Compensate scroll synchronously (in a layout effect, before paint) for prepends —
    including background prepends that don't originate from a user scroll — so the
    viewport stays stable instead of judder-correcting on the next frame.

  ## Markdown rendering
  - Render markdown synchronously *styled* on first paint (paragraphs, lists, code
    cards, tables, inline code) instead of raw escaped text; the async pass then only
    upgrades syntax-highlight colors. Eliminates the flash of full-width raw text.
  - Load KaTeX CSS eagerly with the main bundle instead of inside the lazy markdown
    chunk, avoiding a late stylesheet injection on first render.

  ## Sidebar
  - Hoist per-row recursive tree walks out of row comparators into per-group
    precomputed sets/keys; batch live-session lookups into a single map; add a
    group-level memo boundary.
  - Isolate rename drafts so per-keystroke typing doesn't repaint the row tree.

  ## Sync layer
  - Add a staleness guard so a slow message fetch can't repopulate a session the user
    navigated away from.
  - Throw on fetch failure for authoritative loaders so a transient blip can't read as
    an empty server response.

  ## Cleanup
  - Remove dead code (unused hooks, params, duplicated inline types) surfaced while
    reworking the above.

  ## Known issue
  - A rare, purely cosmetic first-paint width flash can still appear on large sessions;
    it has no behavioral or data impact and is tracked for a follow-up runtime trace.

packages/ui/src/components/session/sidebar/hooks/useProjectSessionSelection.ts

@@ -22,11 +22,9 @@ type Args = {
   openNewSessionDraft: (options?: { directoryOverride?: string | null }) => void;
   setActiveMainTab: (tab: MainTab) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
-  sessions: Session[];
-  worktreeMetadata: Map<string, { path?: string | null }>;
 };
 
-export const useProjectSessionSelection = (args: Args): { currentSessionDirectory: string | null } => {
+export const useProjectSessionSelection = (args: Args): void => {
   const {
     projectSections,
     activeProjectId,
@@ -39,8 +37,6 @@ export const useProjectSessionSelection = (args: Args): { currentSessionDirector
     openNewSessionDraft,
     setActiveMainTab,
     setSessionSwitcherOpen,
-    sessions,
-    worktreeMetadata,
   } = args;
 
   const projectSessionMeta = React.useMemo(() => {
@@ -101,6 +97,7 @@ export const useProjectSessionSelection = (args: Args): { currentSessionDirector
     if (previousActiveProjectRef.current === activeProjectId) {
       return;
     }
+
     const section = projectSections.find((item) => item.project.id === activeProjectId);
     if (!section) {
       return;
@@ -173,20 +170,4 @@ export const useProjectSessionSelection = (args: Args): { currentSessionDirector
     });
   }, [activeProjectId, currentSessionId, projectSessionMeta, setActiveSessionByProject]);
 
-  const currentSessionDirectory = React.useMemo(() => {
-    if (!currentSessionId) {
-      return null;
-    }
-    const metadataPath = worktreeMetadata.get(currentSessionId)?.path;
-    if (metadataPath) {
-      return normalizePath(metadataPath) ?? metadataPath;
-    }
-    const activeSession = sessions.find((session) => session.id === currentSessionId);
-    if (!activeSession) {
-      return null;
-    }
-    return normalizePath((activeSession as Session & { directory?: string | null }).directory ?? null);
-  }, [currentSessionId, sessions, worktreeMetadata]);
-
-  return { currentSessionDirectory };
 };