Polish Settings, Mouse and Scanning forms

[enaboapps]

Closes #886. Stacked on #895, tracked in #891.

What changed

• A setting group that sits directly under a page header, tab row or disclosure no longer draws a second divider.
• Auto scan rate is one stepper: minus, value, plus in a single bordered control with inset focus rings.
• Pointer speed has a real label weight, and the Slow and Fast ends sit closer to the slider.
• Scanner colour and overlay colour choices are chips. The native radios stay in the tree for keyboard and assistive tech, and the chip shows checked and focus states through :has().
• Teaching disclosures show the title in body text and the Show me how action in brand colour.
• Scanning mode cards sit side by side when there is room. A chevron replaces the appended arrow text, and the mode Select starts gets a green status chip. The advanced Menus and Keyboard cards match.
• How scanning works is a bordered disclosure card with a drawn chevron.
• The bottom Scanning note says "Press Select to scan again." and no longer repeats "Saved automatically.", which the status band already shows.

Validation

On this head:

• npm run lint passed
• npm test passed: 238 Vitest tests and 5 Node tests
• npm run build passed
• cargo fmt --check, cargo clippy --all-targets -D warnings and cargo test passed (566 tests, 1 ignored). No Rust changes in this stack.
• Rendered Scanning shared and Point settings, Mouse, and every Settings tab in headless Chrome at 920px and 620px, light and dark.

Review follow-up

An independent review of this layer's head raised these points, now addressed:

• Colour chips in a disabled group no longer show a pointer or hover. The scan rate stepper dims its frame and value when both steps are unavailable.
• A re-review found that hovering the chosen chip in a disabled group hid its fill. The inert hover now skips the checked chip.

🤖 Generated with Claude Code

Rebase validation (2026-09-25)

Rebased onto current main (7fb8fce, including #881) in stack order. git range-diff shows every commit in this PR is patch-equivalent to its previous head, and the intended parent remains its PR base. On the final stack tip (40c75cc), Node 24.19.0 lint, tests and build passed; Rust 1.97.1 format check, Clippy and tests passed. An independent review of the updated PR heads found no actionable findings. CI is rerunning on each new head.