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/commands/page.mdx

@@ -125,7 +125,13 @@ agent-browser set geo <lat> <lng>     # Set geolocation
 agent-browser set offline [on|off]    # Toggle offline mode
 agent-browser set headers <json>      # Extra HTTP headers
 agent-browser set credentials <u> <p> # HTTP basic auth
-agent-browser set media [dark|light]  # Emulate color scheme
+agent-browser set media [dark|light]  # Emulate color scheme (persists for session)
+```
+
+Use `--color-scheme` for persistent dark/light mode across all commands:
+
+```bash
+agent-browser --color-scheme dark open https://example.com
 ```
 
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