feat: implement light mode with settings toggle

Add appearance theme support (dark/light/system) using CSS custom
property overrides. In light mode the Tailwind zinc scale is inverted
and semantic surface/border tokens are swapped, so all existing
utility classes pick up the new palette automatically.

- Add AppearanceTheme type and theme field to WorkspaceSettings
- Add setTheme IPC channel wiring (contracts, preload, handler, service)
- Add [data-theme='light'] CSS overrides for zinc scale, surface tokens,
  scrollbar, and markdown prose colours
- Add Appearance section to SettingsPanel with radio-button theme picker
- Apply data-theme attribute on document root via useEffect in App.tsx,
  with matchMedia listener for the system option

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/channels.ts

@@ -8,6 +8,7 @@ export const ipcChannels = {
   savePattern: 'patterns:save',
   deletePattern: 'patterns:delete',
   setPatternFavorite: 'patterns:set-favorite',
+  setTheme: 'settings:set-theme',
   saveMcpServer: 'tooling:mcp:save',
   deleteMcpServer: 'tooling:mcp:delete',
   saveLspProfile: 'tooling:lsp:save',