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/store.ts

@@ -97,15 +97,83 @@ function quarantineCorrupt(p: string, reason: string): void {
   }
 }
 
-function isValidParsed(v: unknown): v is Partial<AppState> {
+function isValidParsed(v: unknown): v is Record<string, unknown> {
   return typeof v === 'object' && v !== null && !Array.isArray(v)
 }
 
+/**
+ * Current persisted-state schema version. Bump this and add a migration to
+ * MIGRATIONS whenever the on-disk shape changes in a non-additive way.
+ *
+ * Additive changes (new optional fields, new entries in `gamesEnabled`) do
+ * NOT need a version bump — DEFAULT_SETTINGS spread + the `?? []` guards in
+ * `coerce()` handle them gracefully.
+ */
+const CURRENT_SCHEMA_VERSION = 1
+
+type StoredState = Record<string, unknown> & { __schemaVersion?: number }
+
+/**
+ * Migrations are applied in order until the stored version matches CURRENT.
+ * Each fn returns the next-version state. The receiver may freely mutate.
+ *
+ * Note: the v0→v1 migration is a no-op — v1 is the inaugural schema. The
+ * machinery exists so future structural changes (e.g. splitting
+ * `quietHours.days` into a per-window record) have a single explicit place
+ * to live.
+ */
+const MIGRATIONS: Record<number, (s: StoredState) => StoredState> = {
+  0: (s) => s
+}
+
+function runMigrations(s: StoredState): StoredState {
+  let version = typeof s.__schemaVersion === 'number' ? s.__schemaVersion : 0
+  let cursor = s
+  while (version < CURRENT_SCHEMA_VERSION) {
+    const fn = MIGRATIONS[version]
+    if (!fn) {
+      console.warn(
+        `[store] no migration from v${version}; skipping ahead and hoping for the best.`
+      )
+      break
+    }
+    cursor = fn(cursor)
+    version += 1
+  }
+  cursor.__schemaVersion = CURRENT_SCHEMA_VERSION
+  return cursor
+}
+
+/** Coerce a (possibly partial) migrated state into a fully-formed AppState. */
+function coerce(s: StoredState): AppState {
+  return {
+    exercises: Array.isArray(s.exercises) ? (s.exercises as Exercise[]) : [],
+    settings: {
+      ...DEFAULT_SETTINGS,
+      ...(isValidParsed(s.settings) ? (s.settings as Partial<Settings>) : {})
+    },
+    challenges: Array.isArray(s.challenges)
+      ? (s.challenges as Challenge[])
+      : [],
+    gamesEnabled: isValidParsed(s.gamesEnabled)
+      ? (s.gamesEnabled as Partial<Record<GameId, boolean>>)
+      : {},
+    history: Array.isArray(s.history) ? (s.history as HistoryEntry[]) : []
+  }
+}
+
 function load(): AppState {
   const p = getStorePath()
   if (!existsSync(p)) {
     const initial = makeInitial()
-    atomicWrite(p, JSON.stringify(initial, null, 2))
+    atomicWrite(
+      p,
+      JSON.stringify(
+        { __schemaVersion: CURRENT_SCHEMA_VERSION, ...initial },
+        null,
+        2
+      )
+    )
     return initial
   }
   let raw: string
@@ -126,16 +194,7 @@ function load(): AppState {
     quarantineCorrupt(p, `expected object, got ${typeof parsed}`)
     return makeInitial()
   }
-  return {
-    exercises: Array.isArray(parsed.exercises) ? parsed.exercises : [],
-    settings: { ...DEFAULT_SETTINGS, ...(parsed.settings ?? {}) },
-    challenges: Array.isArray(parsed.challenges) ? parsed.challenges : [],
-    gamesEnabled:
-      typeof parsed.gamesEnabled === 'object' && parsed.gamesEnabled !== null
-        ? parsed.gamesEnabled
-        : {},
-    history: Array.isArray(parsed.history) ? parsed.history : []
-  }
+  return coerce(runMigrations(parsed))
 }
 
 function appendHistory(
@@ -207,7 +266,10 @@ function atomicWrite(path: string, contents: string): void {
 
 function flush(): void {
   if (!cache) return
-  atomicWrite(getStorePath(), JSON.stringify(cache, null, 2))
+  // Persist the schema version alongside the state so future migrations know
+  // where to pick up from. The renderer never reads this key.
+  const payload = { __schemaVersion: CURRENT_SCHEMA_VERSION, ...cache }
+  atomicWrite(getStorePath(), JSON.stringify(payload, null, 2))
 }
 
 function scheduleWrite(): void {