fix(cli): respect BROWSER for dashboard launch on Linux

[tsubasakong]

Summary

Fixes #389

• respect the Linux BROWSER environment variable before falling back to xdg-open
• support %s placeholders as well as appended URL arguments so users can pass flags like --no-sandbox
• add focused unit tests for the browser command construction helper

Fixes #389

Changes

• respect the Linux BROWSER environment variable before falling back to xdg-open
• support %s placeholders as well as appended URL arguments so users can pass flags like --no-sandbox
• add focused unit tests for the browser command construction helper

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Testing notes:

• git diff --check
• Rust toolchain is not installed on this host, so compile/test validation is deferred to CI.

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries