fix: address review report across security, architecture, quality, tests

Implements the fixes from docs/REVIEW_REPORT.md (4-agent parallel review).
typecheck clean; 341 tests pass (16 files, +113); build:web ok; coverage
thresholds (80%) enforced in vitest.config.ts.

Critical:
- multi-device approval race: release held approval only when the last
  client detaches (closing one mirror no longer cancels another's prompt)
- unbounded session creation (DoS): Config.maxSessions cap (env MAX_SESSIONS),
  enforced in manager via the existing M4 exit(-1) path
- signal-handler leak: named SIGINT/SIGTERM/uncaughtException refs removed in close()
- terminal-session initialInput timer tracked + cleared on dispose
- tabs.addEntry null-as-cast type hole removed (build session before entry)

Should-fix:
- security-headers middleware + Origin/CSRF guard on DELETE /live-sessions[/:id]
- history.ts converted to fs/promises (async /sessions handler)
- removed dead clientDims map + blur protocol message end-to-end
- per-connection WS message rate limit (Config.maxMsgsPerSec)
- /sessions behavior kept; documented as accepted LAN risk (TECH_DOC §7)

Tests:
- new tmux / preview-grid / terminal-session (jsdom) / tabs (jsdom) suites
- extended history/config/manager/integration coverage incl. regressions

Hygiene:
- parsePositiveInt -> parseNonNegativeInt; ALLOWED_ORIGINS scheme validation
- log-injection sanitize; isLoopback handles 127.0.0.0/8 + IPv4-mapped
- operational constants moved into Config
- extracted public/preview-grid.ts (DRY launcher/manage)
- doc sweeps: ARCHITECTURE §8 runtime-handle exception, stale comments

docs/TECH_DOC.md

@@ -240,6 +240,12 @@ Origin 校验是无认证方案里**唯一不能省**的防线,因为攻击者
 否则 F4(局域网设备访问)会被 F9(Origin 校验)误判 401——二者打架。实现见 ARCHITECTURE §3.1/§3.3。
 前端 WS 的 scheme 随页面协议(`https→wss`),避免 Tailscale/TLS(HTTPS)部署下 `ws://` 被 mixed-content 拦截。
 
+**已知接受风险 —— `/sessions` 端点(O2 历史浏览)**:该路由对局域网内任意设备**无鉴权**返回最近的 Claude Code 会话信息:每个会话的 `cwd`、首条 prompt 的前 ~120 字、以及可 `claude --resume` 的会话 UUID。这是一处真实的信息泄露,但与本应用的威胁模型一致——本应用本身就把完整 shell 交给任何能访问端口的人(无 auth、仅局域网、永不公网)。故**保持现状不加门禁**,通过 Tailscale 部署收敛网络面即可。(此前也评估过用 `/hook` 的 loopback 检查门禁它,但那会使局域网设备无法浏览历史,与 F4 冲突。)代码处有同义注释。
+
+**状态变更路由的 CSRF 守卫**:`DELETE /live-sessions[/:id]`(manage 页 kill/批量 kill)会改变服务端状态,而普通 HTTP 路由不像 WS 升级那样天然有 Origin 检查。故这两条 DELETE 复用同一 `allowedOrigins` 白名单做 Origin 守卫——异源/缺失 Origin → 403——挡住恶意页面无预检触发 Kill-All。配套的安全响应头(`X-Frame-Options: DENY`、`X-Content-Type-Options: nosniff`、保守 CSP)阻止点击劫持。
+
+**连接级限频**:`maxPayload` 只约束单帧**大小**;另加每连接漏桶限频(`MAX_MSGS_PER_SEC`,默认 2000)约束单连接帧**频率**,超限丢帧(不 close,避免误杀合法突发)+ 节流日志。
+
 ---
 
 ## 8. 功能清单