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>

src/main/binaries.ts

@@ -80,7 +80,7 @@ export function getFfprobePath(): string {
   return join(getBinDir(), 'ffprobe.exe')
 }
 
-/** User-facing error shown whenever yt-dlp.exe is not found at startup. */
+/** User-facing error shown by download/probe/index/update when yt-dlp.exe is missing. */
 export const YTDLP_MISSING_MSG =
   'yt-dlp.exe is missing. Open Settings → Software update to re-download it.'