Show pointer speed on an accessible level scale

[enaboapps]

Closes #866.

Replace visible pointer-speed percentages with a Slow–Fast slider showing level 1.0–10.0. Level 5.0 retains the previous default speed. The slider uses logarithmic spacing below and above the default, while Fine tune speed changes the saved speed by its existing 5% step. Mouse scanning and Remote continue sharing the same saved setting and movement behavior. No persisted or protocol schema changes.

The independent review found two accessibility gaps in the first head: a keyboard step could stall on a rounded slider position, and adjacent fine tuning steps could have the same displayed level. The latest head keeps intermediate slider positions, handles keyboard steps directly, and exposes a precise live level in Fine tune speed. Independent re-review of 18ad08be9af3ee73fb0f9d0d854cdb5e9fc15f8f found no remaining actionable issues.

Validation (Node 24.19.0, Rust 1.97.1):

• npm run lint, npm test (223 UI tests, 5 Node tests), npm run build pass on the latest head.
• cargo fmt --manifest-path src-tauri/Cargo.toml --check, cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings, and cargo test --manifest-path src-tauri/Cargo.toml (521 unit tests, 1 ignored, 7 integration tests) pass. Rust was unchanged in the review fix.
• UI tests cover slider mapping, endpoints, older saved values, fine tuning feedback, keyboard progress, navigation, and screen reader naming.
• Built the latest head as a native Windows ARM64 debug app on the running Windows 11 Parallels VM. Visually inspected the Mouse page: Slow–Fast slider, level label, Fine tune disclosure, and repeat controls render as expected with no visible percent. Exercised native WebView keyboard input: Home reached level 1, ArrowRight advanced from the minimum, End restored the previous maximum, and the fine tuning view showed precise feedback with Faster disabled at the maximum. This is a settings UI change; physical switch movement was not retested.