feat: ThemeContext + Settings tabs (Interface / App / Sync)

- ThemeContext: dynamic palette (Default/Giro/Tour/Vuelta), font size
  (small/medium/large), bold labels — all persisted to AsyncStorage
- Settings: three top tabs; Interface tab has palette picker + font
  size pills + bold labels toggle; App tab has km notifications;
  Sync tab has bincio instance login + autarchive placeholder
- RecordingScreen: stat labels now use theme accent colour and scale
  with fontSize; font weight follows boldLabels setting
- All accent/accentDim usages migrated from static colors to useTheme()

src/theme.ts

@@ -9,16 +9,28 @@ export const colors = {
   textMuted:    '#71717a',
   placeholder:  '#52525b',
 
-  accent:       '#60a5fa',
-  accentDim:    'rgba(96,165,250,0.15)',
-
   success:      '#86efac',
   successBg:    '#14532d',
   error:        '#fca5a5',
   errorBg:      '#7f1d1d',
 
-  // recording action buttons
   btnStart:     '#16a34a',
   btnPause:     '#d97706',
   btnStop:      '#dc2626',
 } as const;
+
+export type PaletteKey = 'default' | 'giro' | 'tour' | 'vuelta';
+export type FontSizeKey = 'small' | 'medium' | 'large';
+
+export const PALETTES: Record<PaletteKey, { accent: string; accentDim: string; label: string }> = {
+  default: { accent: '#60a5fa', accentDim: 'rgba(96,165,250,0.15)',  label: 'Default' },
+  giro:    { accent: '#f472b6', accentDim: 'rgba(244,114,182,0.15)', label: "Giro d'Italia" },
+  tour:    { accent: '#facc15', accentDim: 'rgba(250,204,21,0.15)',  label: 'Tour de France' },
+  vuelta:  { accent: '#ef4444', accentDim: 'rgba(239,68,68,0.15)',   label: 'Vuelta a España' },
+};
+
+export const FONT_SCALE: Record<FontSizeKey, number> = {
+  small:  0.88,
+  medium: 1.00,
+  large:  1.15,
+};