Add note labels style dropdown (Alphabetic / Syllabic)

[mevouc]

Summary

• Adds a dropdown next to the "Show note labels" checkbox to switch between Alphabetic (C, D, E...)
  and Syllabic (Do, Re, Mi...) note naming styles
• The selected style is persisted in the user config and updates labels in real time
• Octave numbers now use superscript Unicode characters for a cleaner look (e.g. C⁴ instead of C4)

Changes

• keymap.py: New NoteNameStyletype alias, splitNOTE_NAMESintoALPHA_NOTE_NAMES/SYLLAB_NOTE_NAMES, added OCTAVE_INDEXES` for superscript notation
• settings_store.py: New note_name_style setting with load/save/validation
• main_window.py: Style dropdown (ChevronComboBox) wired to a new noteNameStyleChanged signal
• piano_widget.py: set_note_labels() method to refresh labels dynamically
• app_controller.py: Connects everything, reacts to style changes, regenerates labels, persists the
  setting

[Justagwas]

Thanks for the contribution, the alphabetic/syllabic toggle is something I can get behind.

That said, I do not think this is ready to merge as implemented. C4, A0, etc. follow the standard scientific pitch notation. Changing them to forms like C⁴ introduces a custom display convention that is neither standard SPN nor Helmholtz notation, so it makes the labels less conventional without adding much value. For a standard 88-key piano, the relevant pitch range here is A0 through C8, and I do not think we should deviate from that notation in the UI.

There is also a practical UI issue with the syllabic labels, as they are wider than the current ones, and so, on black keys, they don't fit as well.

I'd support keeping the style toggle, but I think this needs some refinement before merge.