Unify macOS scanning HUD message styling

[enaboapps]

Mac scanning messages now reuse the menu title’s rounded #1E232E panel and centred 18-point white semibold text. Held-switch prompts, point scan escape labels, drag instructions and errors carry explicit internal presentation information. Measured wrapping expands prompt height within the display, preserving full-size error text. Native views and unchanged text/layout are cached.

Windows retains the original prompt geometry and scale. Switch timing, hold-message priority, nonactivating/click-through overlays, settings and BLE are unchanged.

Validation:

• Frontend lint, 149 frontend tests, 5 update-feed tests and production build passed.
• Rust fmt, clippy, 357 unit tests and 7 configuration tests passed.
• Tests cover display-bound height expansion, negative display coordinates, scaling, tiny displays, error presentation metadata and hold/release label restoration.
• Independent review of ffb49fa found no actionable issues.
• Development-signed Mac bundle built successfully. The signed host app was not launched, replaced or interrupted.
• Native Mac VM checks passed on a Retina display: held-switch prompt, Back to rows, hold/release restoration, drag destination, multiline modifier-rejection error and Select to return. A 62-character switch name wraps to two centred lines at full size. TextEdit retained focus during navigation. Small-screen and negative-coordinate bounds are covered by automated tests.
• All CI checks passed, including Mac and Windows packaging, dependency audit and CodeQL.
• Guest settings restored and verified, and VM returned to its prior suspended state. The signed host app remains untouched.

Closes #768