feat: switch stealth Chromium default channel to consumer Chrome

Playwright's bundled Chromium has a distinct build fingerprint (build ID,
uniform WebGL/codec lists, HeadlessChrome residue) that anti-bot services
increasingly flag. Driving a system-installed Google Chrome via Playwright's
channel option sheds that signal and aligns sec-ch-ua with UA more cleanly.

Changes:
- Add BrowserOptions.Channel string field (chrome, chrome-beta, chromium,
  msedge; empty = default).
- When stealth+headless+Chromium and Channel is empty, default to "chrome"
  (was "chromium"). Explicit Channel values always win, so callers can opt
  back to "chromium" or pick another channel.
- Merge Channel in mergeOptions.
- Expose --channel/--ch flag on cmd/browser for A/B fingerprint testing.

Callers must have the chosen browser installed on the host
(e.g. `playwright install chrome`). Firefox and WebKit paths are untouched.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/browser/pkg/browser/flags.go

@@ -46,6 +46,11 @@ var Flags = BrowserFlags{
 		Usage:       "Disable stealth mode (anti-bot-detection evasions are enabled by default)",
 		DefaultText: "false",
 	},
+	&cli.StringFlag{
+		Name:    "channel",
+		Usage:   "Chromium channel to launch (chrome, chrome-beta, chromium, msedge). Only applies when --browser=chromium. Empty = stealth default (chrome).",
+		Aliases: []string{"ch"},
+	},
 }
 
 func FromCommand(ctx context.Context, cmd *cli.Command) (extractor.Browser, error) {
@@ -83,5 +88,9 @@ func FromCommand(ctx context.Context, cmd *cli.Command) (extractor.Browser, erro
 		opts.Stealth = extractor.Bool(false)
 	}
 
+	if ch := cmd.String("channel"); ch != "" {
+		opts.Channel = ch
+	}
+
 	return extractor.NewBrowser(ctx, opts)
 }