feat: theme family system with 12 color palettes + NavRail redesign

Theme system:
- Two-layer architecture: mode (light/dark/system via next-themes) + theme family (color palette via CSS variables)
- 12 theme families: Default, GitHub, Rosé Pine, Tokyo Night, Everforest, Nord, Kanagawa, Night Owl, Poimandres, Vesper, Horizon, SynthWave '84
- Per-family code block highlighting for Prism, HLJS, and Shiki (no more generic oneLight/oneDark fallback)
- Anti-FOUC: synchronous script in <head> reads localStorage and sets data-theme-family before paint
- Theme JSON files in themes/ directory, auto-discovered by server-side loader
- OKLCH color space for all values; CSS specificity html[data-theme-family] beats :root

UI changes:
- NavRail: moved Settings to bottom (replacing theme toggle), removed light/dark toggle button
- Settings: merged General controls into one card with dividers; Appearance section with live Shiki code preview
- Replaced hardcoded blue-500/blue-600 with primary/primary-foreground across 19 components
- Appearance settings: pill mode selector (light/dark/system), theme family dropdown, live code preview

New files:
- src/lib/theme/{types,loader,render-css,context,code-themes}.ts — core theme infrastructure
- src/components/layout/ThemeFamilyProvider.tsx — client context provider
- src/components/settings/AppearanceSection.tsx — appearance settings UI
- src/hooks/useAppTheme.ts — combined theme hook
- themes/*.json — 12 theme family definitions
- docs/handover/theme-system.md — handover documentation

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

themes/github.json

@@ -0,0 +1,80 @@
+{
+  "id": "github",
+  "label": "GitHub",
+  "order": 1,
+  "description": "Clean and familiar, inspired by GitHub's Primer design system",
+  "codeTheme": {
+    "light": "ghcolors",
+    "dark": "vscDarkPlus"
+  },
+  "shikiTheme": {
+    "light": "github-light-default",
+    "dark": "github-dark-default"
+  },
+  "light": {
+    "background": "oklch(1.000 0 0)",
+    "foreground": "oklch(0.254 0.011 254.0)",
+    "card": "oklch(1.000 0 0)",
+    "cardForeground": "oklch(0.254 0.011 254.0)",
+    "popover": "oklch(1.000 0 0)",
+    "popoverForeground": "oklch(0.254 0.011 254.0)",
+    "primary": "oklch(0.540 0.191 257.5)",
+    "primaryForeground": "oklch(1.000 0 0)",
+    "secondary": "oklch(0.947 0.007 247.9)",
+    "secondaryForeground": "oklch(0.338 0.015 252.3)",
+    "muted": "oklch(0.947 0.007 247.9)",
+    "mutedForeground": "oklch(0.531 0.017 251.3)",
+    "accent": "oklch(0.978 0.003 247.9)",
+    "accentForeground": "oklch(0.338 0.015 252.3)",
+    "destructive": "oklch(0.552 0.205 24.5)",
+    "border": "oklch(0.876 0.012 248.0)",
+    "input": "oklch(0.876 0.012 248.0)",
+    "ring": "oklch(0.540 0.191 257.5)",
+    "chart1": "oklch(0.540 0.191 257.5)",
+    "chart2": "oklch(0.546 0.147 146.3)",
+    "chart3": "oklch(0.700 0.160 48.0)",
+    "chart4": "oklch(0.620 0.165 310.0)",
+    "chart5": "oklch(0.600 0.120 185.0)",
+    "sidebar": "oklch(0.978 0.003 247.9)",
+    "sidebarForeground": "oklch(0.254 0.011 254.0)",
+    "sidebarPrimary": "oklch(0.540 0.191 257.5)",
+    "sidebarPrimaryForeground": "oklch(1.000 0 0)",
+    "sidebarAccent": "oklch(0.947 0.007 247.9)",
+    "sidebarAccentForeground": "oklch(0.338 0.015 252.3)",
+    "sidebarBorder": "oklch(0.876 0.012 248.0)",
+    "sidebarRing": "oklch(0.540 0.191 257.5)"
+  },
+  "dark": {
+    "background": "oklch(0.176 0.014 258.4)",
+    "foreground": "oklch(0.943 0.011 243.7)",
+    "card": "oklch(0.220 0.016 256.8)",
+    "cardForeground": "oklch(0.943 0.011 243.7)",
+    "popover": "oklch(0.220 0.016 256.8)",
+    "popoverForeground": "oklch(0.943 0.011 243.7)",
+    "primary": "oklch(0.569 0.202 259.7)",
+    "primaryForeground": "oklch(1.000 0 0)",
+    "secondary": "oklch(0.267 0.015 256.8)",
+    "secondaryForeground": "oklch(0.857 0.014 248.0)",
+    "muted": "oklch(0.267 0.015 256.8)",
+    "mutedForeground": "oklch(0.662 0.018 250.9)",
+    "accent": "oklch(0.246 0.015 256.8)",
+    "accentForeground": "oklch(0.943 0.011 243.7)",
+    "destructive": "oklch(0.665 0.205 27.0)",
+    "border": "oklch(0.330 0.015 252.3)",
+    "input": "oklch(0.330 0.015 252.3)",
+    "ring": "oklch(0.569 0.202 259.7)",
+    "chart1": "oklch(0.569 0.202 259.7)",
+    "chart2": "oklch(0.600 0.140 146.0)",
+    "chart3": "oklch(0.750 0.150 48.0)",
+    "chart4": "oklch(0.680 0.170 310.0)",
+    "chart5": "oklch(0.650 0.120 185.0)",
+    "sidebar": "oklch(0.104 0.019 248.3)",
+    "sidebarForeground": "oklch(0.943 0.011 243.7)",
+    "sidebarPrimary": "oklch(0.569 0.202 259.7)",
+    "sidebarPrimaryForeground": "oklch(1.000 0 0)",
+    "sidebarAccent": "oklch(0.267 0.015 256.8)",
+    "sidebarAccentForeground": "oklch(0.943 0.011 243.7)",
+    "sidebarBorder": "oklch(0.330 0.015 252.3)",
+    "sidebarRing": "oklch(0.569 0.202 259.7)"
+  }
+}