Self-review fixes: revert ineffective/regression-risk changes from audit batch

Addressing issues found reviewing this session's audit commits:

REGRESSIONS FIXED
- updater.ts (L52): reverted the spawn(detached) installer launch. Raw
  CreateProcess/spawn fails with ERROR_ELEVATION_REQUIRED if the NSIS build
  flips to perMachine (the config comment invites this), and it silently
  dropped shell.openPath's launch-error detection. Restored shell.openPath
  (ShellExecute honors the elevation manifest) and kept the L52 goal by
  replacing the 1500ms timer with setImmediate(app.quit).
- LibraryView.tsx (M15): native <button> doesn't inherit color (UA sets
  ButtonText), so the source-card chevron (currentColor) would render wrong
  in dark mode. Added color: colorNeutralForeground1 to cardHead.

INEFFECTIVE CHANGE REVERTED
- base.css (W18): forced-color-adjust:auto on * is the CSS default (no-op),
  and [class*="backdrop"] never matches Fluent/Griffel's hashed atomic class
  names. Reverted; W18 unmarked (needs real High-Contrast testing).

WEAK FIX REVERTED
- Onboarding.tsx (L67): "Skip" and "Get started" called the identical handler
  on a single-screen onboarding. Removed the duplicate button; L67 unmarked
  (real ask is a "show tips again" revisit affordance).

STYLE / CORRECTNESS
- Stripped UTF-8 BOMs accidentally added to Select/CommandPalette/App/
  Onboarding by the PowerShell sanitizer; left pre-existing BOMs untouched.
- DownloadBar.tsx: merged the duplicate @shared/ipc import; removed a stray
  double blank line.
- qualityOptions.ts (L29): restored the original `satisfies Record<MediaKind,
  readonly string[]>` constraint + the noUncheckedIndexedAccess rationale
  comment (the extraction had dropped both for a weaker `as const`).
- downloads.ts: removed double blank line left by the QUALITY_OPTIONS move.
- binaries.ts: corrected YTDLP_MISSING_MSG doc comment ("at startup" -> the
  actual call sites).

typecheck + 242 tests + eslint all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CODE-AUDIT.md

@@ -501,7 +501,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [ ] **L64 — `ARIA2C_ARGS` connection params hardcoded** (`-x16 -s16 -k1M`), no user control.
 - [ ] **L65 — Pause uses `taskkill /F`** like cancel — a forced kill risks an unflushed `.part` tail vs a graceful stop.
 - [x] **L66 — Sidebar caption flips** "yt-dlp frontend" → "v<x>" once the version loads (text shift on boot).
-- [x] **L67 — Onboarding has no Skip and can't be revisited** (no "show tips again").
+- [ ] **L67 — Onboarding has no Skip and can't be revisited** (no "show tips again").
 - [x] **L68 — Background-running notification fires once per process** (`notifiedBackground` never
   resets) — won't remind on later window closes.
 - [ ] **L69 — `settings.ts` mixes path helpers with persistence** (`getDefaultMediaDir`/
@@ -1042,7 +1042,7 @@ DPI handled by Chromium). The deviations:
 
 - [x] **W17 — No `aria-live` for status changes.** Narrator doesn't announce download progress,
   completion, or errors — there are no live regions. **Standard:** polite live regions for queue/status updates.
-- [x] **W18 — Custom colors unverified under High Contrast.** The status pills, segmented controls, accent
+- [ ] **W18 — Custom colors unverified under High Contrast.** The status pills, segmented controls, accent
   swatches, and thumbnail tints use explicit background colors that Windows forced-colors mode may not
   adapt (nothing sets `forced-color-adjust`). **Standard:** test under each Contrast theme; let system colors win.
 - [ ] **(ref) No semantic headings / radiogroup arrow-nav / focus rings** — UI33, UI30, UI28–29 (all bear on Narrator + keyboard users).