feat: add --allow-file-access flag for file:// URL support (#375)

* feat: add --allow-file-access flag for file:// URL support

Adds the ability to open and interact with local files using file:// URLs.
This enables use cases like viewing local PDFs, testing local HTML files,
and allowing JavaScript to access other local files via XHR.

The flag adds Chromium's --allow-file-access-from-files and --allow-file-access
launch arguments. Only supported in Chromium browsers.

Fixes #345

* fix: add cli_allow_file_access tracking to prevent spurious warning

When --allow-file-access is set via AGENT_BROWSER_ALLOW_FILE_ACCESS env var
(not CLI), don't warn about the flag being ignored when daemon is already running.

cli/src/output.rs

@@ -1771,6 +1771,7 @@ Options:
   --proxy-bypass <hosts>     Bypass proxy for these hosts (or AGENT_BROWSER_PROXY_BYPASS)
                              e.g., --proxy-bypass "localhost,*.internal.com"
   --ignore-https-errors      Ignore HTTPS certificate errors
+  --allow-file-access        Allow file:// URLs to access local files (Chromium only)
   -p, --provider <name>      Browser provider: ios, browserbase, kernel, browseruse
   --device <name>            iOS device name (e.g., "iPhone 15 Pro")
   --json                     JSON output