feat: add per-key notes with keyboard tooltips

[mondalaci]

Summary

• Add per-key notes in the remap popover (sticky-note toggle, auto-growing textarea) and persist them via trailing KeyLabelAction serialization
• Show a top-right corner marker on keys that have notes, with an optional SVG data-note-marker-transform for non-rectangular keys
• Display a compact hover tooltip with the note text on the keymap editor

Closes #2289

Test plan

• Open a key remap popover, toggle the note button, confirm the textarea appears focused and the button tooltip switches between “Add a note to this key.” / “Remove note.”
• Enter a multi-line note, remap the key, reopen the popover and confirm the note is restored
• Confirm keys with notes show a rounded corner marker and hovering shows a centered tooltip with an upward arrow
• Toggle the note off, remap, and confirm the marker/tooltip disappear
• Save config to the device (or export/import) and verify notes survive round-trip
• Check Modifiers ↔ Secondary role spacing matches Scancode ↔ Modifiers spacing

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