feat(themes): add Vision Dark & Vision Navy colorblind-safe themes (closes #166)

Purple & Gold palette (Deuteranopia / Protanopia / Tritanopia safe).
Vision Dark: near-black #0D0B12 + Gold #FFD700 (~14.7:1 WCAG AAA).
Vision Navy: deep navy #0A1628 + Gold #FFD700 (~14.5:1 WCAG AAA).
New 'Accessibility' group in ThemePicker.
Removed stale theme count from aboutFeatures across all locales.

This is an initial step toward colorblind accessibility — color variables
alone cannot cover all CVD use cases. Structural improvements (secondary
indicators, pattern/shape cues) are still needed in future iterations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/ThemePicker.tsx

@@ -123,6 +123,13 @@ export const THEME_GROUPS: { group: string; themes: ThemeDef[] }[] = [
       { id: 'powerslave',            label: 'Powerslave (inspired)',            bg: '#F0DFB0', card: '#2A1808', accent: '#C8960C' },
     ],
   },
+  {
+    group: 'Accessibility',
+    themes: [
+      { id: 'vision-dark',  label: 'Vision Dark',  bg: '#0d0b12', card: '#16131e', accent: '#ffd700' },
+      { id: 'vision-navy',  label: 'Vision Navy',  bg: '#0a1628', card: '#112038', accent: '#ffd700' },
+    ],
+  },
   {
     group: 'Social Media',
     themes: [