feat: pro-instrument desktop UI revamp - #256

Redesign the desktop app shell and core surfaces into a cohesive
"pro instrument" visual language (Figma/Linear/Raycast-grade),
replacing the generic shadcn/dashboard look. See DESIGN.md.

- Tokens: graphite blue-black surfaces + one electric-indigo accent,
  tighter radii, dark-by-default; remove the 8-color accent-picker
  sprawl. Add apps/desktop-ui/DESIGN.md as the north star.
- Tabs: Figma-style square tabs (rounded top, dividers, welded active
  tab + content seam) replacing pills.
- Unified top bar: brand + live app-version chip + theme/settings/
  profile; moved out of the sidebar, which is now pure navigation.
  Extract useAppUser / useSignOut hooks; remove dead sidebar code.
- Native window chrome: OS title bar off, macOS traffic lights inset
  (titleBarStyle overlay + trafficLightPosition); grant
  core:window:allow-start-dragging so the custom titlebar drags.
- Sidebar nav + Cmd-K command palette flattened to the one-accent
  language (no gradient pills / stray violet).
- Dashboard: light-touch reskin (solid greeting, flat graphite panels).
- Tool template: shared ToolPageHeader + JSON Formatter + Regex Tester
  aligned to the pro surface/toolbar vocabulary.
- Fix pnpm packageManager pin to 11.13.1 (11.12.0 is a broken release
  that crashes pnpm in-repo with an "integrity" error).

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