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>

docs/handover/README.md

@@ -11,3 +11,4 @@
 | agent-tooling-todo-bridge.md | SDK → SSE → DB 事件流、TodoWrite 字段映射、去重策略 |
 | bridge-system.md | 多 IM 远程桥接系统架构（目录结构、数据流、设计决策） |
 | assistant-workspace.md | 助理工作区：人格/记忆文件、对话式引导、自动触发、确定性落盘 |
+| theme-system.md | 主题家族系统：两层架构、JSON schema、代码高亮三条渲染链、12 个主题清单 |

docs/handover/theme-system.md

@@ -0,0 +1,82 @@
+# Theme Family System / 主题家族系统
+
+## 概述
+
+两层主题系统：`mode`（light/dark/system，由 next-themes 管理）+ `theme family`（色板，由自定义 context + CSS 变量管理）。
+
+## 架构
+
+```
+themes/*.json           → 定义色板（OKLCH）、代码主题映射
+src/lib/theme/loader.ts → 服务端读取 + 校验 JSON，按 order 排序
+src/lib/theme/render-css.ts → 生成 html[data-theme-family="x"] 规则
+src/app/layout.tsx      → <head> 注入反闪烁脚本 + <style> 块
+src/lib/theme/context.ts + ThemeFamilyProvider → 客户端状态 + localStorage
+```
+
+**CSS 特异性**：`html[data-theme-family]` (0,1,0) > `:root` (0,0,1)；`html.dark[data-theme-family]` (0,2,0) > `.dark` (0,1,0)。globals.css 中的 `:root` / `.dark` 块作为兜底。
+
+## 主题 JSON Schema
+
+```jsonc
+{
+  "id": "night-owl",          // 唯一 ID，对应 data-theme-family 值
+  "label": "Night Owl",       // 显示名
+  "order": 7,                 // 排序权重
+  "description": "...",       // 可选短描述
+  "codeTheme": {              // Prism / HLJS 代码高亮
+    "light": "coy",           // 键名对应 code-themes.ts 中的 PRISM_THEME_MAP / HLJS_THEME_MAP
+    "dark": "nightOwl"
+  },
+  "shikiTheme": {             // Shiki 代码高亮（ai-elements）
+    "light": "night-owl-light",  // 值为 Shiki BundledTheme 名
+    "dark": "night-owl"
+  },
+  "light": { /* 31 个 ThemeColors 键，oklch 值 */ },
+  "dark":  { /* 同上 */ }
+}
+```
+
+## 代码高亮三条渲染链
+
+| 消费组件 | 高亮器 | 解析函数 |
+|---------|--------|---------|
+| ChatCodeBlock (`CodeBlock.tsx`) | Prism | `resolvePrismStyle()` |
+| DocPreview / FilePreview | HLJS | `resolveHljsStyle()` |
+| AI Elements CodeBlock (`code-block.tsx`) | Shiki | `resolveShikiThemes()` |
+| Settings Preview (`AppearanceSection.tsx`) | Shiki | `codeToHtml()` |
+
+三条链路都通过 `useThemeFamily()` 取得当前 family，再查找对应的 `codeTheme` / `shikiTheme` 映射。
+
+## 当前主题清单（12 个）
+
+| ID | Order | Light Code | Dark Code | Shiki Light | Shiki Dark |
+|----|-------|-----------|----------|------------|-----------|
+| default | 0 | oneLight | oneDark | github-light | github-dark |
+| github | 1 | ghcolors | vscDarkPlus | github-light-default | github-dark-default |
+| rose-pine | 2 | duotoneLight | duotoneSea | rose-pine-dawn | rose-pine |
+| tokyo-night | 3 | materialLight | materialOceanic | github-light | tokyo-night |
+| everforest | 4 | gruvboxLight | duotoneForest | everforest-light | everforest-dark |
+| nord | 5 | coldarkCold | nord | github-light | nord |
+| kanagawa | 6 | vs | duotoneEarth | kanagawa-lotus | kanagawa-wave |
+| night-owl | 7 | coy | nightOwl | night-owl-light | night-owl |
+| poimandres | 8 | coldarkCold | coldarkDark | min-light | poimandres |
+| vesper | 9 | gruvboxLight | twilight | min-light | vesper |
+| horizon | 10 | duotoneLight | materialDark | github-light | horizon |
+| synthwave84 | 11 | prism | synthwave84 | min-light | synthwave-84 |
+
+## 新增主题
+
+1. 在 `themes/` 目录创建 JSON 文件，遵循上述 schema
+2. 填写所有 31 个 ThemeColors 键（light + dark），值用 OKLCH
+3. `codeTheme` 键名必须在 `code-themes.ts` 的 `PRISM_THEME_MAP` / `HLJS_THEME_MAP` 中存在
+4. `shikiTheme` 值必须是 Shiki `BundledTheme`
+5. Loader 自动发现——无需改代码
+
+## 关键设计决策
+
+- **不用 Catppuccin / Solarized / Monokai 占位**：已删除早期占位主题，只保留有完整 light+dark 对且在两种模式下都有明显差异的 family
+- **Rose Pine 合并**：Dawn 是 light 变体，Base 是 dark 变体，Moon 的 light 和 Base 一样 → 只保留一个 rose-pine family
+- **Tokyo Night 合并**：Storm 和 Base 在 light 模式几乎无差异 → 只保留一个 tokyo-night family，light 使用真实 TN Light 色板
+- **硬编码蓝色清理**：全局 `blue-*` / `indigo-*` Tailwind 类已替换为 `primary` 语义色，保留语义状态色（green/red/yellow）
+- **Settings 预览用 Shiki**：与 ai-elements 实际渲染一致，而非 Prism 近似