add --color-scheme flag for persistent dark/light mode (#528)

Fixes #519. Playwright defaults `colorScheme` to `light` on all new contexts, overriding the browser/OS dark mode setting. This is especially disruptive in CDP mode, where every reconnection resets the scheme. The `set media dark` command also didn't persist its choice to new tabs or pages.

- Add `--color-scheme <dark|light|no-preference>` flag, config key (`colorScheme`), and env var (`AGENT_BROWSER_COLOR_SCHEME`)
- Store the preference in `BrowserManager` and automatically apply it to all new contexts (via Playwright's context option) and all new pages (via `page.emulateMedia` in `setupPageTracking`)
- `set media dark/light` now also persists its choice for subsequent pages and tabs

docs/src/app/configuration/page.mdx

@@ -72,6 +72,7 @@ Every CLI flag can be set in the config file using its camelCase equivalent:
     <tr><td><code>allowFileAccess</code></td><td><code>--allow-file-access</code></td><td>boolean</td></tr>
     <tr><td><code>cdp</code></td><td><code>--cdp</code></td><td>string</td></tr>
     <tr><td><code>autoConnect</code></td><td><code>--auto-connect</code></td><td>boolean</td></tr>
+    <tr><td><code>colorScheme</code></td><td><code>--color-scheme</code></td><td>string (<code>dark</code>, <code>light</code>, <code>no-preference</code>)</td></tr>
     <tr><td><code>headers</code></td><td><code>--headers</code></td><td>string (JSON)</td></tr>
   </tbody>
 </table>