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/ipc.ts

@@ -7,6 +7,7 @@ import type {
   LspProfileDefinition,
   McpServerDefinition,
   SessionToolingSelection,
+  AppearanceTheme,
 } from '@shared/domain/tooling';
 import type { WorkspaceState } from '@shared/domain/workspace';
 
@@ -92,6 +93,7 @@ export interface ElectronApi {
   selectPattern(patternId?: string): Promise<WorkspaceState>;
   selectSession(sessionId?: string): Promise<WorkspaceState>;
   setPatternFavorite(input: SetPatternFavoriteInput): Promise<WorkspaceState>;
+  setTheme(theme: AppearanceTheme): Promise<WorkspaceState>;
   onWorkspaceUpdated(listener: (workspace: WorkspaceState) => void): () => void;
   onSessionEvent(listener: (event: SessionEventRecord) => void): () => void;
 }