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

src/http/history.ts

@@ -6,7 +6,7 @@
  * can `claude --resume <id>` in the right place). Read-only; best-effort.
  */
 
-import fs from 'node:fs';
+import fs from 'node:fs/promises';
 import os from 'node:os';
 import path from 'node:path';
 
@@ -61,27 +61,27 @@ export function parseSessionMeta(jsonlText: string): { cwd: string | null; previ
   return { cwd, preview: preview.replace(/\s+/g, ' ').trim().slice(0, 120) };
 }
 
-function readHead(file: string, max: number): string {
+/** Read up to `max` bytes from the head of a file (async, best-effort). */
+async function readHead(file: string, max: number): Promise<string> {
+  let fh: fs.FileHandle | undefined;
   try {
-    const fd = fs.openSync(file, 'r');
-    try {
-      const buf = Buffer.alloc(max);
-      const n = fs.readSync(fd, buf, 0, max, 0);
-      return buf.subarray(0, n).toString('utf8');
-    } finally {
-      fs.closeSync(fd);
-    }
+    fh = await fs.open(file, 'r');
+    const buf = Buffer.alloc(max);
+    const { bytesRead } = await fh.read(buf, 0, max, 0);
+    return buf.subarray(0, bytesRead).toString('utf8');
   } catch {
     return '';
+  } finally {
+    await fh?.close().catch(() => undefined);
   }
 }
 
-/** Most-recently-modified Claude Code sessions across all projects. */
-export function listSessions(limit = 50): HistorySession[] {
+/** Most-recently-modified Claude Code sessions across all projects (async). */
+export async function listSessions(limit = 50): Promise<HistorySession[]> {
   const root = path.join(os.homedir(), '.claude', 'projects');
   let dirs: string[];
   try {
-    dirs = fs.readdirSync(root);
+    dirs = await fs.readdir(root);
   } catch {
     return [];
   }
@@ -91,14 +91,14 @@ export function listSessions(limit = 50): HistorySession[] {
     const dirPath = path.join(root, dir);
     let names: string[];
     try {
-      names = fs.readdirSync(dirPath);
+      names = await fs.readdir(dirPath);
     } catch {
       continue;
     }
     for (const name of names) {
       if (!name.endsWith('.jsonl')) continue;
       try {
-        const st = fs.statSync(path.join(dirPath, name));
+        const st = await fs.stat(path.join(dirPath, name));
         files.push({
           id: name.slice(0, -'.jsonl'.length),
           file: path.join(dirPath, name),
@@ -112,10 +112,13 @@ export function listSessions(limit = 50): HistorySession[] {
 
   files.sort((a, b) => b.mtimeMs - a.mtimeMs);
 
-  return files.slice(0, limit).map((f) => {
-    const meta = parseSessionMeta(readHead(f.file, 256 * 1024));
-    const cwd = meta.cwd ?? '';
-    const project = cwd !== '' ? (cwd.split('/').filter(Boolean).pop() ?? cwd) : 'unknown';
-    return { id: f.id, cwd, project, mtimeMs: f.mtimeMs, preview: meta.preview };
-  });
+  const top = files.slice(0, limit);
+  return Promise.all(
+    top.map(async (f) => {
+      const meta = parseSessionMeta(await readHead(f.file, 256 * 1024));
+      const cwd = meta.cwd ?? '';
+      const project = cwd !== '' ? (cwd.split('/').filter(Boolean).pop() ?? cwd) : 'unknown';
+      return { id: f.id, cwd, project, mtimeMs: f.mtimeMs, preview: meta.preview };
+    }),
+  );
 }