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/preload/index.ts

@@ -13,6 +13,7 @@ const api: ElectronApi = {
   savePattern: (input) => ipcRenderer.invoke(ipcChannels.savePattern, input),
   deletePattern: (patternId) => ipcRenderer.invoke(ipcChannels.deletePattern, patternId),
   setPatternFavorite: (input) => ipcRenderer.invoke(ipcChannels.setPatternFavorite, input),
+  setTheme: (theme) => ipcRenderer.invoke(ipcChannels.setTheme, theme),
   saveMcpServer: (input) => ipcRenderer.invoke(ipcChannels.saveMcpServer, input),
   deleteMcpServer: (serverId) => ipcRenderer.invoke(ipcChannels.deleteMcpServer, serverId),
   saveLspProfile: (input) => ipcRenderer.invoke(ipcChannels.saveLspProfile, input),