perf(ui): improve mobile session switching

Treat the mobile web surface as a constrained runtime so sync loads smaller message pages, keeps fewer warm session caches, and evicts heavy inactive sessions instead of retaining them across switches.

Limit mobile message-record and turn-model caches to reduce memory pressure on phones while preserving bounded initial page expansion for large final turns.

Split the mobile session status bar so the collapsed state avoids subscribing to the full session/status list; the expensive grouping work now only mounts for the expanded list.

Verified with bun run type-check and bun run lint.

packages/ui/src/components/chat/hooks/useChatTimelineController.ts

@@ -14,6 +14,7 @@ import {
 import type { TurnHistorySignals } from '../lib/turns/historySignals';
 import { getMemoryLimits, type SessionHistoryMeta } from '@/stores/types/sessionTypes';
 import { isVSCodeRuntime } from '@/lib/desktop';
+import { isMobileSurfaceRuntime } from '@/lib/runtimeSurface';
 
 type ViewportAnchor = { messageId: string; offsetTop: number };
 
@@ -63,12 +64,19 @@ export interface UseChatTimelineControllerResult {
 const TURN_MODEL_CACHE_MAX = 30
 const VSCODE_TURN_MODEL_CACHE_MAX = 4
 const VSCODE_TURN_MODEL_CACHE_MAX_MESSAGES = 30
+const MOBILE_TURN_MODEL_CACHE_MAX = 4
+const MOBILE_TURN_MODEL_CACHE_MAX_MESSAGES = 30
 const turnModelCache = new Map<string, { messages: ChatMessageEntry[]; model: TurnWindowModel }>()
-const getTurnModelCacheMax = () => isVSCodeRuntime() ? VSCODE_TURN_MODEL_CACHE_MAX : TURN_MODEL_CACHE_MAX
+const getTurnModelCacheMax = () => {
+    if (isVSCodeRuntime()) return VSCODE_TURN_MODEL_CACHE_MAX
+    if (isMobileSurfaceRuntime()) return MOBILE_TURN_MODEL_CACHE_MAX
+    return TURN_MODEL_CACHE_MAX
+}
 
 const shouldCacheTurnModelMessages = (messages: ChatMessageEntry[]): boolean => {
-    if (!isVSCodeRuntime()) return true
-    return messages.length <= VSCODE_TURN_MODEL_CACHE_MAX_MESSAGES
+    if (isVSCodeRuntime()) return messages.length <= VSCODE_TURN_MODEL_CACHE_MAX_MESSAGES
+    if (isMobileSurfaceRuntime()) return messages.length <= MOBILE_TURN_MODEL_CACHE_MAX_MESSAGES
+    return true
 }
 
 const rememberTurnModel = (key: string, value: { messages: ChatMessageEntry[]; model: TurnWindowModel }) => {