chore+fix: repo hygiene, code-review fixes, audit cleanup

Three independent code reviews + a security audit produced ~200 findings.
This commit lands the high-impact subset. Tests pass (53), typecheck
clean, eslint clean (3 minor exhaustive-deps warnings left).

REPO HYGIENE
- Add .editorconfig, .prettierrc.json, .prettierignore.
- Add ESLint flat config (.eslintrc.cjs) — correctness-focused, no style
  rules (Prettier owns formatting).
- Add `format` / `format:check` / `lint` npm scripts.
- Add CHANGELOG.md (Keep a Changelog format, back-filled to 0.1.x).
- Reformat all source via Prettier so future diffs stay small.

DATA SAFETY (src/main/store.ts)
- Atomic write (tmp + rename) with retry on transient EBUSY/EPERM —
  was non-atomic writeFileSync, vulnerable to truncation on power loss.
- On corrupt JSON, rename to `app-state.json.corrupt-<ts>` instead of
  silently overwriting the user's exercises/history with defaults.
- Validate parsed shape before merging — reject arrays/scalars where
  objects expected; per-field array checks.
- Strip `id` from incoming patches in updateExercise/updateChallenge —
  a runtime caller (IPC) could otherwise smuggle id changes through.
- clearHistory now refuses an unbounded wipe (no beforeTs => no-op);
  callers must pass an explicit boundary.
- unref() the debounce timer so it doesn't keep the event loop alive.

SECURITY (src/main/*)
- gsi-server: hard 256 KB body cap (was unbounded — local OOM vector),
  reject any Origin/Sec-Fetch-Site header (blocks browser CSRF from
  visited pages), require application/json Content-Type, generic 400
  on parse error (no error string echo to client), closeAllConnections
  + async close on stop.
- dota2: validate auth.token from payload with timingSafeEqual against
  the per-install token — was unauthenticated, any local process could
  forge match-end events. Narrow object shape before spread-merge to
  avoid throws on hostile payloads like {player:"x"}. Reset latest /
  prevState after match_end so the next match starts clean.
- ipc: gate `dev:simulateMatchEnd` registration behind `!app.isPackaged`
  so it does not exist in shipped builds.
- preload: gate the matching `simulateMatchEnd` export behind
  `import.meta.env.MODE !== 'production'` so the bundler dead-code-
  eliminates it from the production preload bundle.
- windows: shell.openExternal allowlist (http/https/mailto only) — was
  forwarding any URL, including file:/javascript:/custom URI handlers
  (some Windows handlers have been RCE vectors). will-navigate blocks
  navigation to anywhere except file:// or the dev URL.

CORRECTNESS (src/main/* + src/shared/*)
- shared/types.ts isQuietAt: fix wrap-around + day-of-week filter.
  With from=22:00 to=07:00 days=[Mon..Fri], the window started THE
  PREVIOUS DAY when we're in the AM half — old code checked today's
  day-of-week and got the wrong answer Sat 02:00 and Mon 01:00. Now
  the filter is evaluated against the window's START day. Also reject
  malformed HH:MM strings instead of producing NaN.
- scheduler: call broadcastState() after firing exercises so the
  renderer's Dashboard/Exercises pages don't show stale nextFireAt
  until the next state-changing IPC. Guard powerMonitor listeners
  against double-registration on dev hot-reload.
- dota2: fix `launchOptionStatus = steamRunning ? 'queued' : 'queued'`
  tautology — both branches now correctly read 'queued'.
- steam-launch-options: replace `require('node:fs')` inside atomicWrite
  with the top-level import; retry on transient EBUSY/EPERM.

CORRECTNESS (src/renderer/*)
- lib/history.ts: replace `today.getTime() - i * MS_DAY` arithmetic
  with `setDate(date - i)` calendar arithmetic in dailyRepsRange and
  currentStreak — DST transitions shift epoch math by ±1h and cause
  dayKey() to emit duplicate or missing days at the boundary.
- lib/icon.tsx: restrict name lookup to ICON_CHOICES set — an arbitrary
  string from a corrupted state file could otherwise resolve to
  unrelated Lucide exports and crash the renderer.
- lib/format.ts: guard formatCountdown against NaN/Infinity.
- i18n/index.ts: replace regex-based interpolation with split/join so
  variable values containing regex metacharacters interpolate
  literally; warn in dev on missing keys; clamp pluralRu(-N) via abs.
- ReminderApp: keyboard shortcuts moved INTO ExerciseReminder so Enter
  respects the stepper's `adjusted` flag (was always passing planned
  reps). Stepper capped at 5× planned. Don't hijack Space when a
  button is focused. `key={exercise.id+nextFireAt}` forces a fresh
  component for back-to-back reminders so stepper state resets. Match
  summary view gets Esc-to-close. Functional setMode in onMarkDone
  avoids races against stale `mode.done`.
- UpdaterCard: guard against NaN/Infinity in download-progress events
  (electron-updater fires early events with undefined fields).
- Games: gate DevPanel behind `import.meta.env.DEV` in addition to the
  main-side IPC gate, and narrow the `simulateMatchEnd` access.
- Add aria-labels for the +/- stepper buttons (i18n keys added).

TESTS
- +2 quiet-hours tests covering wrap-around + day-filter combo and
  malformed HH:MM fallback. Total 53 passing.

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

src/preload/index.ts

@@ -17,7 +17,8 @@ type Unsub = () => void
 type Handler<T> = (payload: T) => void
 
 function on<T>(channel: string, handler: Handler<T>): Unsub {
-  const listener = (_e: Electron.IpcRendererEvent, payload: T): void => handler(payload)
+  const listener = (_e: Electron.IpcRendererEvent, payload: T): void =>
+    handler(payload)
   ipcRenderer.on(channel, listener)
   return () => ipcRenderer.removeListener(channel, listener)
 }
@@ -44,7 +45,8 @@ const api = {
     ipcRenderer.invoke(IPC.updateSettings, patch),
 
   getAccentColor: (): Promise<string> => ipcRenderer.invoke(IPC.getAccentColor),
-  getOsTheme: (): Promise<'light' | 'dark'> => ipcRenderer.invoke(IPC.getOsTheme),
+  getOsTheme: (): Promise<'light' | 'dark'> =>
+    ipcRenderer.invoke(IPC.getOsTheme),
 
   pauseAll: (): Promise<void> => ipcRenderer.invoke(IPC.pauseAll),
   resumeAll: (): Promise<void> => ipcRenderer.invoke(IPC.resumeAll),
@@ -69,17 +71,31 @@ const api = {
   // Challenges
   addChallenge: (input: Omit<Challenge, 'id'>): Promise<Challenge> =>
     ipcRenderer.invoke(IPC.addChallenge, input),
-  updateChallenge: (id: string, patch: Partial<Challenge>): Promise<Challenge> =>
-    ipcRenderer.invoke(IPC.updateChallenge, id, patch),
+  updateChallenge: (
+    id: string,
+    patch: Partial<Challenge>
+  ): Promise<Challenge> => ipcRenderer.invoke(IPC.updateChallenge, id, patch),
   deleteChallenge: (id: string): Promise<boolean> =>
     ipcRenderer.invoke(IPC.deleteChallenge, id),
   toggleChallenge: (id: string, enabled: boolean): Promise<Challenge> =>
     ipcRenderer.invoke(IPC.toggleChallenge, id, enabled),
 
-  closeMatchSummary: (): Promise<void> => ipcRenderer.invoke(IPC.closeMatchSummary),
+  closeMatchSummary: (): Promise<void> =>
+    ipcRenderer.invoke(IPC.closeMatchSummary),
 
-  simulateMatchEnd: (id: GameId, stats: Record<string, number>): Promise<void> =>
-    ipcRenderer.invoke('dev:simulateMatchEnd', id, stats),
+  // Dev-only: synthesize a match-end event from the renderer. The channel is
+  // not registered in production builds (see src/main/ipc.ts), so this
+  // function will reject in shipped binaries even though it's exposed.
+  // Gated at the preload level too so the bundler can dead-code-eliminate it.
+  ...(import.meta.env.MODE !== 'production'
+    ? {
+        simulateMatchEnd: (
+          id: GameId,
+          stats: Record<string, number>
+        ): Promise<void> =>
+          ipcRenderer.invoke('dev:simulateMatchEnd', id, stats)
+      }
+    : {}),
 
   // Auto-updater
   updaterStatus: (): Promise<UpdaterStatus> =>
@@ -99,9 +115,11 @@ const api = {
   onFire: (h: Handler<Exercise>): Unsub => on(IPC.evtFire, h),
   onMatchEnd: (h: Handler<MatchSummary>): Unsub => on(IPC.evtMatchEnd, h),
   onStateChanged: (h: Handler<AppState>): Unsub => on(IPC.evtStateChanged, h),
-  onThemeChanged: (h: Handler<'light' | 'dark'>): Unsub => on(IPC.evtThemeChanged, h),
+  onThemeChanged: (h: Handler<'light' | 'dark'>): Unsub =>
+    on(IPC.evtThemeChanged, h),
   onAccentChanged: (h: Handler<string>): Unsub => on(IPC.evtAccentChanged, h),
-  onGamesChanged: (h: Handler<GameStatus[]>): Unsub => on(IPC.evtGamesChanged, h),
+  onGamesChanged: (h: Handler<GameStatus[]>): Unsub =>
+    on(IPC.evtGamesChanged, h),
   onUpdaterStatus: (h: Handler<UpdaterStatus>): Unsub =>
     on(IPC.evtUpdaterStatus, h)
 }