Dark mode contrast, tuner display and misc UI fixes

- color-scheme per theme so native controls (selects, checkboxes, number
  inputs, sliders) render dark-native instead of black-on-black.
- Themed .control-select text color; new .control-input class replaces
  hardcoded white-on-black inline input styles.
- Tuner widget stays mounted when enabled: idle state (dimmed, needle parked,
  '—') instead of appearing/disappearing. Fully theme-var based (was
  hardcoded white — invisible in light mode).
- Remove CSS transitions on per-frame values (tuner needle, level meter):
  transitions interrupted 60x/sec are left stuck by WebKit/Safari; the
  display EMA from the detection loop replaces them.
- VexFlow notation: resolve CSS var() to concrete colors (SVG attributes
  can't resolve var(), silently fell back to black) and override VexFlow's
  hardcoded #444 ledger-line default; re-render on theme changes
  (useLayoutEffect so children read the updated attribute in time).
- Themed switch toggles (were hardcoded #ddd / white — glowed in dark mode).
- Fixed-height pitch readout so the layout stops jumping when a note starts
  or stops sounding.
- Split SettingsContext into context/Provider/hook files (fast-refresh);
  memoized provider functions; typed webkitAudioContext; prefer-const;
  removed 'as any' casts. ESLint now passes clean.

src/components/PianoKeys.tsx

@@ -145,7 +145,7 @@ export function PianoKeys({
         // Sort specifically so black keys are last (on top in SVG z-order)
         return rects.sort((a, b) => (a.isBlack === b.isBlack) ? 0 : a.isBlack ? 1 : -1);
 
-    }, [keys, whiteKeyWidth, viewMode]);
+    }, [keys, whiteKeyWidth]);
 
 
     return (