Share one scan panel layout between keyboard and mouse

[enaboapps]

Closes #860.

The keyboard's panel layout now lives in a new scan_panel module: panel size, padding, weighted rows with even gaps, the status tile and docking. The keyboard and the Mouse panel only supply their keys and status text.

• Keyboard: output is unchanged. Before the refactor I recorded its frames with a one-off test across several states (row scan, key select, docked top, word predictions, error) and three screen/scale combinations. The refactored code reproduces them byte for byte (788 KB of frames).
• Mouse panel: now matches the keyboard's size, padding and gaps, and replaces the floating label with a status tile. The status reads "Movement · Select a row" / "Select ↖", with paused, back-to-rows, failed and moving-pointer messages, and back-to-rows is highlighted like on the keyboard. Direction keys use the keyboard's character colour; More, Movement, Keyboard, Dock and Switch to Point use its toolbar colour.
• Workflow: the moving-pointer prompt is now passed into the panel instead of being patched onto the frame afterwards.
• Out of scope: the action menu keeps its current layout.

Validation:

• Node 24: npm run lint, npm test (218 UI tests, 5 Node tests) and npm run build pass.
• Rust 1.97.1: cargo fmt --check, cargo clippy --all-targets -- -D warnings and cargo test pass (517 unit tests, 1 ignored, 7 integration tests). New tests cover the shared layout (padding, gaps, weights, status tile, docking, highlighting) and the Mouse panel (keyboard geometry, status wording, moving and failed states).
• I compared composites of the tile backgrounds before and after (text is drawn natively). The real overlay has not been checked on macOS or Windows yet.
• Independent review of 4f47d65 found no blocking issues and two minor ones. Both are fixed in fe46f8a: the speed keys now use two-line labels ("Slower / 100%"), so the status reads "More controls · Select Slower 100%" instead of having two separators, and a workflow test now checks the moving prompt with nothing highlighted. Independent review of latest head fe46f8a found no actionable findings. Rust checks re-run on fe46f8a: fmt, clippy and 517 unit tests pass.
• All PR checks pass on fe46f8a: frontend, dependency audit, macOS native, Windows native/NSIS, CodeQL, and JavaScript/TypeScript analysis.

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