[fix] Keep Launch visible with a compact action area

[Fail-Safe]

The start menu caps its content height at 832 points, so permission prompts and longer settings can push Launch below the visible area. Size the window from its laid-out content up to the display's available height, and keep Launch and the secondary actions outside the scrolling settings area.

Launch remains full-width. Center Reset beneath it, with the existing linked attribution aligned to Launch's right edge on the same row. This reduces the fixed action area's height while keeping Reset below Launch. Settings retain their scroll position when the window is sized, and accessibility reading order follows settings then actions.

Example:

This includes the button-order commit from #186. If #186 merges first, that portion will drop out of this diff. The Return-key launch action, reset confirmation, permission behavior, and VM settings are unchanged.

Validation:

• Full make test passed under the installed Xcode toolchain: 243 native tests, 103 guest tests, and shell/storage contracts. The optional virtio-pinch guest ABI test was skipped because its test binary was not configured.
• Layout checks cover display heights of 500, 700, 900, and 1400 points with permissions ungranted: actions remain visible and stationary during scrolling, the window fits the screen, the taller layout fits its content, Reset is centered under Launch, and attribution is right-aligned without overlapping Reset.
• An isolated native preview using the changed source was visually checked: Launch, centered Reset, and the right-aligned attribution remain visible in the fixed action area. The preview used inert action callbacks and did not launch or reset a VM.
• git diff --check passed.

This PR has not been installed into the daily-use app bundle.