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/SessionFolderItem.tsx

@@ -3,6 +3,7 @@ import { cn } from '@/lib/utils';
 import type { SessionFolder } from '@/stores/useSessionFoldersStore';
 import { useI18n } from '@/lib/i18n';
 import { Icon } from "@/components/icon/Icon";
+import type { SessionNodeChildRenderExtras, SessionNodeRenderExtras } from './sidebar/sessionNodeItemUtils';
 
 interface SessionFolderItemProps<TSessionNode> {
   folder: SessionFolder;
@@ -19,7 +20,17 @@ interface SessionFolderItemProps<TSessionNode> {
     groupDir?: string | null,
     projectId?: string | null,
     archivedBucket?: boolean,
+    secondaryMeta?: { projectLabel?: string | null; branchLabel?: string | null } | null,
+    renderContext?: 'project' | 'recent',
+    renderExtras?: SessionNodeChildRenderExtras,
   ) => React.ReactNode;
+  /**
+   * Returns the precomputed per-row render extras for a given node. The
+   * group precomputes subtree-contains lookups once, then resolves a
+   * per-node structure key here so SessionNodeItem's React.memo comparator
+   * can answer with a single string compare instead of a recursive walk.
+   */
+  getRenderExtras?: (node: TSessionNode) => SessionNodeRenderExtras<TSessionNode> | undefined;
   groupDirectory?: string | null;
   projectId?: string | null;
   mobileVariant?: boolean;
@@ -54,6 +65,7 @@ const SessionFolderItemBase = <TSessionNode,>({
   onRename,
   onDelete,
   renderSessionNode,
+  getRenderExtras,
   groupDirectory,
   projectId,
   mobileVariant = false,
@@ -320,7 +332,7 @@ const SessionFolderItemBase = <TSessionNode,>({
           {/* Then sessions */}
           {sessions.length > 0 ? (
             sessions.map((node) =>
-              renderSessionNode(node, 0, groupDirectory ?? null, projectId ?? null, archivedBucket),
+              renderSessionNode(node, 0, groupDirectory ?? null, projectId ?? null, archivedBucket, undefined, 'project', getRenderExtras?.(node)),
             )
           ) : !subFolderItems ? (
             <div className="py-1 pl-1.5 text-left typography-micro text-muted-foreground/70">