Improve scancode selector with tiled category layout

[mondalaci]

Summary

• Replace the keypress-tab ng-select scancode dropdown with a dedicated scancode-select panel that lists categories vertically in a narrow, viewport-tall overlay.
• Tile short-label categories (Letter, Number, Punctuation, Whitespace, Function, Numpad) to cut scrolling; Numpad uses fixed rows (0–4, 5–9, operators, Enter/NumLock).
• Keep incremental filtering and custom M/B/S scancode tags; raise overlay z-index so the panel appears above the key-action popover.

Closes #2288

Test plan

• Open a key action popover on a keymap layer → Scancode dropdown opens above the popover (not behind it)
• Open a macro key action → panel does not cover the trigger; search field is usable at the top of the panel
• Confirm tiled categories: Letter, Number, Punctuation, Whitespace, Function, Numpad
• Confirm Numpad row order: 0–4, 5–9, / * - + ., Enter NumLock
• Type to filter (e.g. media, esc) and confirm empty categories disappear
• Add a custom scancode via M123 / B45 / S12
• Capture keystroke still updates the selected scancode
• Check light and dark themes for hover/selected tile styles

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