perf: overhaul session loading, caching, and runtime isolation (#2360)

Improve OpenChamber responsiveness under large session workloads while fixing
cache, synchronization, and persistence correctness across runtimes, projects,
directories, and worktrees.

- prioritize selected and visible sessions during bootstrap and defer
  non-critical enrichment work
- reduce redundant message loading, event processing, store publication, and
  hidden sidebar work
- prevent stale session and message requests from overwriting newer
  authoritative state
- preserve existing data when authoritative fetches fail instead of treating
  failures as successful empty responses
- scope session materialization, messages, drafts, queues, todos, pins,
  permissions, folders, tabs, Git state, and pull request data by runtime and
  directory identity
- harden runtime switching, reconnect, cleanup, mutation reconciliation, and
  persisted-state ordering
- preserve live subagent Task linkage when metadata arrives after an older
  message request or while streaming parts are suspended
- coalesce overlapping tail refreshes without losing newer refresh demand
- improve cold-session loading by moving deferrable work out of the critical
  bootstrap path
- isolate URL authentication, mobile credentials, native secrets, and other
  runtime-owned state across endpoint changes
- bound long-lived caches and remove avoidable allocations from event and
  rendering hot paths
- limit virtualization to archive collections where it improves rendering
  without disrupting active sidebar layout
- stabilize session folders, pin ordering, expanded state, and persisted
  sidebar behavior
- open skill files through the same secure editor and outside-workspace grant
  flow used by file navigation, including worktree sessions
- expand regression coverage for stale completions, runtime collisions,
  reconnect behavior, persistence races, authoritative empty results, and
  subagent refresh ordering
- document the updated synchronization, cache ownership, performance, and
  runtime-isolation invariants

packages/web/src/api/settings.ts

@@ -5,9 +5,7 @@ const SETTINGS_ENDPOINT = '/api/config/settings';
 const RELOAD_ENDPOINT = '/api/config/reload';
 
 const sanitizePayload = (data: unknown): SettingsPayload => {
-  if (!data || typeof data !== 'object') {
-    return {};
-  }
+  if (!data || typeof data !== 'object' || Array.isArray(data)) throw new Error('Invalid settings response');
   return data as SettingsPayload;
 };
 
@@ -22,7 +20,7 @@ export const createWebSettingsAPI = (): SettingsAPI => ({
       throw new Error(`Failed to load settings: ${response.statusText}`);
     }
 
-    const payload = sanitizePayload(await response.json().catch(() => ({})));
+    const payload = sanitizePayload(await response.json());
     return {
       settings: payload,
       source: 'web',
@@ -44,7 +42,7 @@ export const createWebSettingsAPI = (): SettingsAPI => ({
       throw new Error(error.error || 'Failed to save settings');
     }
 
-    const payload = sanitizePayload(await response.json().catch(() => ({})));
+    const payload = sanitizePayload(await response.json());
     return payload;
   },