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>

tests/shared/tooling.test.ts

@@ -53,6 +53,7 @@ describe('tooling settings helpers', () => {
     });
 
     expect(workspaceSettings).toEqual({
+      theme: 'dark',
       tooling: {
         mcpServers: [
           {

tests/shared/workspace.test.ts

@@ -9,6 +9,7 @@ describe('workspace seed', () => {
     expect(workspace.projects).toEqual([]);
     expect(workspace.sessions).toEqual([]);
     expect(workspace.settings).toEqual({
+      theme: 'dark',
       tooling: {
         mcpServers: [],
         lspProfiles: [],