feat: a11y + Error Boundary + IPC validation + schema migrations

Second pass through the audit punch-list. ESLint and Prettier now clean
(0 errors, 0 warnings), typecheck clean, 53 tests pass.

ACCESSIBILITY (Modal)
- Full focus trap: Tab/Shift-Tab cycle within the dialog and never
  escape to the underlying page.
- Focus restoration: closing returns focus to the trigger button.
- First focusable child is focused on open (skipping the X button).
- aria-labelledby links the dialog to its <h2> via useId().
- Close button's hardcoded "Закрыть" replaced with i18n key.

ERROR RECOVERY
- Add ErrorBoundary component (class — only way) with localized
  fallback and a "try again" reset button. Stack trace shown only in
  dev. Wrapped around the whole App + a nested boundary around the
  routed pages so a crash in one route doesn't blank the chrome.
- Module-level guard on subscribeToBackend so React 18 StrictMode's
  dev-mode double-mount doesn't subscribe twice.
- Loading placeholder is now blank (was hardcoded Russian "Загрузка…"
  that English users would see during initial hydration).

TRAY i18n
- 5 tray strings now follow the current settings.language. Falls back
  to Russian when the store isn't loaded yet or the lang is unknown.
- refreshMenu() called on settings.language change and on
  pauseAll/resumeAll so the pause label stays in sync with state.

IPC VALIDATION (src/main/validate.ts)
- Hand-rolled validators for every renderer-supplied payload:
  exercise input/patch, challenge input/patch, settings patch, id,
  actualReps, snoozeMinutes. Range-check numeric fields
  (intervalMinutes ∈ [1, 1440], reps ∈ [1, 9999], multiplier ∈ [0,
  1000], snooze ∈ [1, 1440]), cap string lengths at 200, restrict
  enums (theme/lang/notify-mode/stat) to known values, validate
  quietHours.from/to with HH:MM regex and dedup quietHours.days.
- Every ipcMain.handle for mutations now runs the validator first and
  returns null on rejection instead of pushing junk into the store.
  A compromised renderer can no longer corrupt persisted state via
  out-of-range numbers or wrong-type fields.

SCHEMA MIGRATIONS (src/main/store.ts)
- Add __schemaVersion field to persisted state with CURRENT = 1.
- MIGRATIONS map: { 0: (s) => s } as a no-op seed; future structural
  changes (e.g. quietHours shape revision) get a single explicit slot.
- runMigrations() applies migrations in order; coerce() normalises the
  result into a fully-formed AppState. Both first-write and every
  flush() persist the version field.

EXHAUSTIVE-DEPS WARNINGS
- Dashboard: memoise `exercises` so downstream useMemos don't fire on
  every parent render; gate the history fetch on exercises change
  instead of any state change.
- HistoryHeatmap: wrap `weeks` in useMemo so monthLabels' deps are
  stable.

LINT POLISH
- updater.ts: refactor a Prettier-vs-no-extra-semi conflict by
  extracting the cast into a local binding.
- Remove dead import of `Challenge` from ipc.ts (now imported via
  validators).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main/updater.ts

@@ -26,7 +26,8 @@ function setStatus(s: UpdaterStatus): void {
   // Preserve lastCheckedAt across status transitions where applicable.
   if (s.kind === 'not-available' || s.kind === 'idle') {
     if (lastCheckedAt && !('lastCheckedAt' in s)) {
-      ;(s as { lastCheckedAt?: number }).lastCheckedAt = lastCheckedAt
+      const withTs = s as { lastCheckedAt?: number }
+      withTs.lastCheckedAt = lastCheckedAt
     }
   }
   currentStatus = s