feat(audit): Batch 14 — perf (code-split + seed DCE) + first-run default

- PERF8: code-split the non-default tabs. Library/History/Terminal/Settings are
  React.lazy chunks behind a Suspense; Downloads (the launch tab) stays eager. The
  production build now emits separate SettingsView (~90kB), LibraryView (~29kB),
  HistoryView (~16kB), TerminalView (~8kB) chunks — ~140kB out of the initial
  bundle. The stores these views use are imported eagerly elsewhere, so lazy-
  loading the views never delays store startup/persistence (per the C2 note).
- L128: gate the three preview seed arrays (downloadSeed / sources seeds / history
  seed) on `import.meta.env.DEV && PREVIEW`. In the packaged build DEV is
  statically false, so Rollup constant-folds and dead-code-eliminates the seed
  literals; the browser dev preview (DEV true) still gets them.
- SR4: clipboardWatch now defaults false for new installs (privacy — no clipboard
  read on focus until opted in). Existing installs keep their saved value
  (electron-store fills only missing keys); the onboarding tip points to the
  setting. NOTE: this flips a signature convenience off-by-default for new users —
  flagged for the maintainer, trivially reversible.

Deferred with rationale (CODE-AUDIT.md): PERF3 (core-queue micro-opt on a bounded
list, risky unattended), PERF6/L162 (bounded by virtualization + conflicts with
PERF5's hoisted renderItem), SR5 (MP3 = deliberate compatibility default; the label
bug was M18), W12 (fallback-of-a-fallback icon), W15 (GPU software-rendering
workaround — external driver dependency), L104 (History virtualization needs the
L161 layout rework + live verification; bounded by the 500-cap).

Verified: typecheck (node+web) + 279 tests + eslint + production build green
(code-split chunks confirmed in output); touched files prettier-clean.

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

src/shared/ipc.ts

@@ -638,7 +638,10 @@ export const DEFAULT_SETTINGS: Settings = {
   // greeted by a bright white window despite full system-theme support.
   theme: 'system',
   accentColor: 'teal',
-  clipboardWatch: true,
+  // Off on first launch (SR4): reading the clipboard on every window focus is a
+  // privacy surprise for a brand-new user. It's a one-toggle opt-in in Settings
+  // (the onboarding tip points there). Existing installs keep their saved value.
+  clipboardWatch: false,
   downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
   proxy: '',
   rateLimit: '',