browser: integrate navigation and input QoL fixes

[hongnoul]

Summary

• suppress false failure overlays for cancelled history navigation
• allow non-answer keys through browser confirmation prompts
• default new windows to half-width while preserving preferred_width configuration
• keep user-activated desktop links visible inside agent environments

Validation

• cargo fmt --all --check
• cargo test --workspace (252 passed, 1 ignored)

[hongnoul]

Review verdict: changes requested (comment-only self-review).

P1 – inherited GIO markers can turn agent opens into visible windows. default_open_mode() treats the mere presence of GIO_LAUNCHED_DESKTOP_FILE as proof that this specific URL activation was user-initiated. That environment variable is inherited by descendants of any desktop-launched application, including a desktop-launched terminal or agent UI, so later agent calls can incorrectly bypass agent/headless mode. GIO also supplies GIO_LAUNCHED_DESKTOP_FILE_PID; require it to match the current process (and ideally verify the desktop file is Hwatu's), or rely on the explicit one-shot JCODE_OPEN_ORIGIN=user marker. Add a test for an inherited marker from a different PID.

The cancellation, confirmation-key propagation, and half-width changes otherwise look sound. Please also update docs/agents.md, which still says new windows use one third of the first monitor width.

Validation: all CI checks are green. I independently ran the focused open-origin and confirmation-key tests successfully.