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/main/EryxAppService.ts

@@ -51,6 +51,8 @@ import {
 } from '@shared/domain/runTimeline';
 import {
   createSessionToolingSelection,
+  normalizeTheme,
+  type AppearanceTheme,
   type LspProfileDefinition,
   type McpServerDefinition,
   normalizeLspProfileDefinition,
@@ -201,6 +203,12 @@ export class EryxAppService extends EventEmitter<AppServiceEvents> {
     return this.persistAndBroadcast(workspace);
   }
 
+  async setTheme(theme: AppearanceTheme): Promise<WorkspaceState> {
+    const workspace = await this.loadWorkspace();
+    workspace.settings.theme = normalizeTheme(theme);
+    return this.persistAndBroadcast(workspace);
+  }
+
   async deletePattern(patternId: string): Promise<WorkspaceState> {
     if (isBuiltinPattern(patternId)) {
       throw new Error('Built-in patterns cannot be deleted.');

src/main/ipc/registerIpcHandlers.ts

@@ -16,6 +16,7 @@ import type {
   UpdateScratchpadSessionConfigInput,
 } from '@shared/contracts/ipc';
 import type { QuerySessionsInput } from '@shared/domain/sessionLibrary';
+import type { AppearanceTheme } from '@shared/domain/tooling';
 
 import { EryxAppService } from '@main/EryxAppService';
 
@@ -33,6 +34,9 @@ export function registerIpcHandlers(window: BrowserWindow, service: EryxAppServi
   ipcMain.handle(ipcChannels.setPatternFavorite, (_event, input: SetPatternFavoriteInput) =>
     service.setPatternFavorite(input.patternId, input.isFavorite),
   );
+  ipcMain.handle(ipcChannels.setTheme, (_event, theme: AppearanceTheme) =>
+    service.setTheme(theme),
+  );
   ipcMain.handle(ipcChannels.saveMcpServer, (_event, input: SaveMcpServerInput) =>
     service.saveMcpServer(input.server),
   );