feat: refactor composer to two-layer layout, add session permission & context indicator

Composer restructure:
- MessageInput internal footer now only contains: file button, model
  selector, slash command button, and send button
- Removed mode toggle (Code/Plan) from composer area (logic preserved)
- New ChatComposerActionBar below input with three slots: left (Design
  Agent), center (permission selector), right (context indicator)

New components:
- SlashCommandButton: inserts `/` at cursor, reuses existing slash
  command detection and popover logic
- ChatPermissionSelector: session-level permission toggle (default /
  full_access) with danger confirmation dialog
- ContextUsageIndicator: SVG donut chart showing context usage ratio
  with HoverCard displaying model, tokens, cache stats
- ChatComposerActionBar: slot-based container for action bar items

Session permission (permission_profile):
- DB: new column on chat_sessions, default 'default'
- API: PATCH sessions supports updating permission_profile
- Chat API: bypassPermissions = global setting OR session full_access
- PermissionPrompt: auto-approves and hides when full_access
- Claude client: combines global and per-session bypass flags

Bridge integration:
- permission-broker: auto-approves permissions for full_access sessions
  without sending IM notifications
- conversation-engine: passes bypassPermissions to SDK for full_access
- PATCH route: auto-approves pending bridge permissions on switch to
  full_access

Other:
- ImageGenToggle relabeled to "Design Agent" / "设计 Agent"
- New model-context.ts with MODEL_CONTEXT_WINDOWS lookup
- Provider models API returns optional contextWindow field
- New useContextUsage hook for token usage calculation
- i18n: added composer.*, permission.*, context.* keys (en + zh)

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

docs/exec-plans/active/composer-refactor.md

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+# Composer 重构 + 单聊天权限 + 远程桥接联动
+
+## 状态: 🟡 进行中
+
+## 概述
+重构聊天输入区为两层结构，新增 session 级权限档位，桥接联动，上下文占用指示器。
+
+## 轨道拆分
+
+### Track 1: Backend & Data（数据层）
+- [ ] db.ts: chat_sessions 新增 permission_profile TEXT NOT NULL DEFAULT 'default'
+- [ ] types/index.ts: Session 类型新增 permission_profile 字段
+- [ ] api/chat/sessions/route.ts: 创建 session 时写入 default
+- [ ] api/chat/sessions/[id]/route.ts: GET 返回 permission_profile, PATCH 支持更新
+- [ ] api/chat/route.ts: bypassPermissions = global || session.permission_profile === 'full_access'
+- [ ] lib/model-context.ts: 新建模型 contextWindow 常量表
+- [ ] api/providers/models/route.ts: 返回可选 contextWindow 字段
+- [ ] i18n/en.ts + zh.ts: 新增所有翻译 key
+
+### Track 2: Composer UI 重构
+- [ ] MessageInput.tsx: 内部 footer 只保留文件、模型、slash、发送
+- [ ] 移除 mode toggle（保留后端逻辑）
+- [ ] 新建 ChatComposerActionBar.tsx: 输入框外部操作栏容器
+- [ ] 新建 SlashCommandButton.tsx: 复用现有 slash 逻辑
+- [ ] ImageGenToggle 改文案"设计 Agent"，移到 action bar
+- [ ] ChatView.tsx + page.tsx: 集成新布局
+
+### Track 3: 权限选择器 + 上下文指示器
+- [ ] 新建 ChatPermissionSelector.tsx: default/full_access 切换 + 危险确认
+- [ ] 新建 ContextUsageIndicator.tsx: 圆圈 + HoverCard
+- [ ] 新建 hooks/useContextUsage.ts: 解析 usage、计算 ratio
+- [ ] 集成到 ChatComposerActionBar
+
+### Track 4: Bridge 联动
+- [ ] permission-broker.ts: 检查 session permission_profile
+- [ ] bridge-manager.ts: full_access 时不发权限通知
+- [ ] conversation-engine.ts: full_access 时跳过权限卡片
+- [ ] 处理切换时已存在的 pending permission 失效
+
+## 依赖关系
+- Track 2/3/4 都依赖 Track 1 的类型和 DB 字段
+- Track 2 和 Track 3 可并行，最后在 ChatView 集成
+- Track 4 独立于 UI 轨道
+
+## 测试清单
+- [ ] session 级 permission_profile 持久化
+- [ ] slash 按钮触发现有命令流程
+- [ ] full_access 下本地不显示权限确认
+- [ ] full_access 下 bridge 不发权限通知
+- [ ] context hover 展示明细
+- [ ] composer 布局回归