Refactor/harmony

[azfoo]

• Roman numeral figured bass — dynamically computes MusAnalysis figured-bass suffixes for all chord qualities and inversions using music21's chord structure, replacing the previous hard-coded triad/seventh table. Handles triads, seventh, ninth, eleventh, and thirteenth chords at any inversion; extended chords drop implied intermediates (e.g. the 7th in a 9th chord). Augmented-sixth, suspended, and other bespoke qualities keep a static table.
• Modal scale types — mode labels now display the mode name in a readable font alongside the tonic (e.g. "C Dorian", "B♭ Phrygian") rather than rendering the mode name through the MusAnalysis glyph font.
• Quality-aware inversion clamping — when changing a chord's quality in the inspector or the add-harmony dialog, the inversion is clamped to the maximum valid for the new quality. Closes Crash when switching to chord that has no 3rd inversion #468.
• Chord symbol fallback for uninvertible qualities — letter_symbol now catches ChordException and falls back to root position when music21 cannot construct the requested inversion (e.g. minor-sixth at inversion 3). Closes 3rd Inversion of Minor 6th chord crashes the app #376.
• Add-harmony dialog sync — selecting a note/quality/accidental from the combo boxes now writes the chord symbol back into the text field, so the two entry methods stay in sync.
• Refactor — harmony constants (NOTE_NAME_TO_INT, INT_TO_NOTE_NAME, etc.) are now derived from music21 at import time rather than duplicated by hand; _INV_TO_STRING is the single source of truth for inversion labels across the dialog and inspector.