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

cli/src/output.rs

@@ -2062,6 +2062,7 @@ Options:
   --headed                   Show browser window (not headless)
   --cdp <port>               Connect via CDP (Chrome DevTools Protocol)
   --auto-connect             Auto-discover and connect to running Chrome
+  --color-scheme <scheme>    Color scheme: dark, light, no-preference (or AGENT_BROWSER_COLOR_SCHEME)
   --session-name <name>      Auto-save/restore session state (cookies, localStorage)
   --config <path>            Use a custom config file (or AGENT_BROWSER_CONFIG env)
   --debug                    Debug output
@@ -2103,6 +2104,7 @@ Environment:
   AGENT_BROWSER_PROVIDER         Browser provider (ios, browserbase, kernel, browseruse)
   AGENT_BROWSER_AUTO_CONNECT     Auto-discover and connect to running Chrome
   AGENT_BROWSER_ALLOW_FILE_ACCESS Allow file:// URLs to access local files
+  AGENT_BROWSER_COLOR_SCHEME     Color scheme preference (dark, light, no-preference)
   AGENT_BROWSER_STREAM_PORT      Enable WebSocket streaming on port (e.g., 9223)
   AGENT_BROWSER_IOS_DEVICE       Default iOS device name
   AGENT_BROWSER_IOS_UDID         Default iOS device UDID
@@ -2126,6 +2128,7 @@ Examples:
   agent-browser wait --load networkidle  # Wait for slow pages to load
   agent-browser --cdp 9222 snapshot      # Connect via CDP port
   agent-browser --auto-connect snapshot  # Auto-discover running Chrome
+  agent-browser --color-scheme dark open example.com  # Dark mode
   agent-browser --profile ~/.myapp open example.com    # Persistent profile
   agent-browser --session-name myapp open example.com  # Auto-save/restore state