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/tray.ts

@@ -3,9 +3,45 @@ import { join } from 'node:path'
 import { showMainWindow } from './windows'
 import { isPaused, setPaused, forceCheck } from './scheduler'
 import { snoozeAll } from './state-actions'
+import { getSettings } from './store'
+import type { Language } from '@shared/types'
 
 let tray: Tray | null = null
 
+/**
+ * Minimal tray-side localisation. The renderer's full i18n dict lives in
+ * `src/renderer/src/i18n/dict.ts` and isn't reachable from the main process
+ * tsconfig, so we keep the 5 strings the tray actually uses here.
+ */
+const TRAY_STRINGS: Record<Language, Record<string, string>> = {
+  ru: {
+    open: 'Открыть',
+    pause: 'Пауза напоминаний',
+    resume: 'Возобновить напоминания',
+    snooze15: 'Отложить все на 15 мин',
+    quit: 'Выход'
+  },
+  en: {
+    open: 'Open',
+    pause: 'Pause reminders',
+    resume: 'Resume reminders',
+    snooze15: 'Snooze all 15 min',
+    quit: 'Quit'
+  }
+}
+
+function trayLabel(key: string): string {
+  // getSettings reads from cache; if the store hasn't loaded yet (very early
+  // boot) it lazily reads from disk. Defaults to 'ru' if anything goes wrong.
+  let lang: Language = 'ru'
+  try {
+    lang = getSettings().language ?? 'ru'
+  } catch {
+    /* keep default */
+  }
+  return TRAY_STRINGS[lang]?.[key] ?? TRAY_STRINGS.ru[key] ?? key
+}
+
 function resolveTrayIcon(): Electron.NativeImage {
   // Try resources/, fallback to a transparent 16x16 if missing during dev.
   const candidates = [
@@ -35,10 +71,10 @@ export function refreshMenu(): void {
   if (!tray) return
   const paused = isPaused()
   const menu = Menu.buildFromTemplate([
-    { label: 'Открыть', click: () => showMainWindow() },
+    { label: trayLabel('open'), click: () => showMainWindow() },
     { type: 'separator' },
     {
-      label: paused ? 'Возобновить напоминания' : 'Пауза напоминаний',
+      label: paused ? trayLabel('resume') : trayLabel('pause'),
       click: () => {
         setPaused(!paused)
         refreshMenu()
@@ -46,12 +82,12 @@ export function refreshMenu(): void {
       }
     },
     {
-      label: 'Отложить все на 15 мин',
+      label: trayLabel('snooze15'),
       click: () => snoozeAll(15)
     },
     { type: 'separator' },
     {
-      label: 'Выход',
+      label: trayLabel('quit'),
       click: () => {
         app.quit()
       }