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/ui/src/lib/i18n/messages/uk.ts

@@ -1838,6 +1838,12 @@ export const dict: Record<I18nKey, string> = {
   "chat.container.returnToParent.title": "Повернутися до батьківської сесії",
   "chat.container.returnToParent.label": "Батьківська",
   "chat.container.readOnlySubagentPromptBanner": "Сесії субагентів не можна запитувати.",
+  "chat.container.sessionLoadError.title": "Не вдалося завантажити сесію",
+  "chat.container.sessionLoadError.description": "Перевірте з’єднання та спробуйте завантажити цю сесію ще раз.",
+  "chat.container.sessionLoadError.retry": "Спробувати знову",
+  "sessions.sidebar.group.empty.loadingSessions": "Завантаження сесій…",
+  "sessions.sidebar.group.empty.loadFailed": "Не вдалося оновити сесії.",
+  "sessions.sidebar.group.empty.retry": "Спробувати знову",
   "chat.unifiedControls.title": "Елементи керування",
   "chat.unifiedControls.model.title": "Модель",
   "chat.unifiedControls.model.noRecent": "Немає останніх моделей",