[codex] Support custom settings on Settings page

[cormoran]

Summary

• Add the cormoran/zmk-feature-custom-settings RPC schema and a useCustomSettings hook for listing, memory-writing, saving, discarding, and resetting scoped settings.
• Show detected custom settings after Power Management on the Settings page, grouped by owning custom subsystem and setting key.
• Display split-side values together and let memory updates target All, Central, or a specific Peripheral.
• Render editors for bool, int32, string, bytes, options, arrays, layer IDs, and HID usages; layer IDs use layer dropdowns and HID usages use the existing keycode picker UI.
• Mark pending memory changes in the value section and enable scoped Save/Discard/Reset controls per custom subsystem.
• Extend demo mode with sample range, option, layer, HID, split, pending, save, discard, and reset behavior.

Validation

• npm run format:check
• npm run lint
• npm test -- --runInBand
• npm run build
• Verified Settings page in demo mode at http://127.0.0.1:5173/ with split custom-setting values, layer dropdowns, HID keycode picker, pending marks, and subsystem Save/Discard/Reset controls.

[github-actions]

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_{Deployed from commit 06d8143}