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/main.rs

@@ -358,6 +358,10 @@ fn main() {
             launch_cmd["ignoreHTTPSErrors"] = json!(true);
         }
 
+        if let Some(ref cs) = flags.color_scheme {
+            launch_cmd["colorScheme"] = json!(cs);
+        }
+
         let err = match send_command(launch_cmd, &flags.session) {
             Ok(resp) if resp.success => None,
             Ok(resp) => Some(
@@ -440,6 +444,10 @@ fn main() {
             launch_cmd["ignoreHTTPSErrors"] = json!(true);
         }
 
+        if let Some(ref cs) = flags.color_scheme {
+            launch_cmd["colorScheme"] = json!(cs);
+        }
+
         let err = match send_command(launch_cmd, &flags.session) {
             Ok(resp) if resp.success => None,
             Ok(resp) => Some(
@@ -461,12 +469,16 @@ fn main() {
 
     // Launch with cloud provider if -p flag is set
     if let Some(ref provider) = flags.provider {
-        let launch_cmd = json!({
+        let mut launch_cmd = json!({
             "id": gen_id(),
             "action": "launch",
             "provider": provider
         });
 
+        if let Some(ref cs) = flags.color_scheme {
+            launch_cmd["colorScheme"] = json!(cs);
+        }
+
         let err = match send_command(launch_cmd, &flags.session) {
             Ok(resp) if resp.success => None,
             Ok(resp) => Some(
@@ -494,7 +506,8 @@ fn main() {
         || flags.proxy.is_some()
         || flags.args.is_some()
         || flags.user_agent.is_some()
-        || flags.allow_file_access)
+        || flags.allow_file_access
+        || flags.color_scheme.is_some())
         && flags.cdp.is_none()
         && flags.provider.is_none()
     {
@@ -556,6 +569,10 @@ fn main() {
             launch_cmd["allowFileAccess"] = json!(true);
         }
 
+        if let Some(ref cs) = flags.color_scheme {
+            launch_cmd["colorScheme"] = json!(cs);
+        }
+
         match send_command(launch_cmd, &flags.session) {
             Ok(resp) if !resp.success => {
                 // Launch command failed (e.g., invalid state file, profile error)