feat(multimonitor): header button to open Codeman spanned across all displays

Replace the header notification bell (now hidden by default; still reachable
via Settings → Notifications and the drawer) with a multi-monitor button.
Clicking it POSTs /api/system/span-displays, which spawns the bundled
scripts/span-codeman.sh — a fresh, maximized browser --app window sized to the
union of all displays — so in-page floating session panels can be dragged
across the physical monitor seam. macOS only; needs the one-time "Displays
have separate Spaces" OFF prerequisite (documented in the script). The route
pins the spanned window to localhost with a digits-only port from the Host
header so nothing attacker-controllable reaches the launched browser.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -105,7 +105,8 @@
           </div>
         </div>
         <button class="btn-icon-header btn-response-viewer-header" onclick="app.toggleResponseViewer()" title="View last response" aria-label="View last response"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z"/><circle cx="12" cy="12" r="3"/></svg></button>
-        <button class="btn-icon-header btn-notifications" onclick="app.toggleNotifications()" title="Notifications" aria-label="Toggle notifications">
+        <button class="btn-icon-header btn-multimonitor" onclick="app.launchMultiMonitor()" title="Open Codeman across all displays" aria-label="Open Codeman across all displays"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg></button>
+        <button class="btn-icon-header btn-notifications" onclick="app.toggleNotifications()" title="Notifications" aria-label="Toggle notifications" style="display:none;">
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 8A6 6 0 0 0 6 8c0 7-3 9-3 9h18s-3-2-3-9"/><path d="M13.73 21a2 2 0 0 1-3.46 0"/></svg>
           <span class="notification-badge" id="notifBadge" style="display:none;">0</span>
         </button>

src/web/public/panels-ui.js

@@ -3129,6 +3129,23 @@ Object.assign(CodemanApp.prototype, {
     this.notificationManager?.toggleDrawer();
   },
 
+  // Open a Codeman window stretched across all displays (multi-monitor mode).
+  // The server spawns scripts/span-codeman.sh, which launches a fresh, spanning
+  // browser --app window so in-page floating panels can cross the monitor seam.
+  async launchMultiMonitor() {
+    try {
+      const res = await fetch('/api/system/span-displays', { method: 'POST' });
+      const data = await res.json().catch(() => ({}));
+      if (res.ok && data.success) {
+        this.showToast('Opening Codeman across all displays…', 'success');
+      } else {
+        this.showToast(data.error || 'Could not open spanning window', 'error');
+      }
+    } catch (err) {
+      this.showToast('Could not open spanning window: ' + (err?.message || err), 'error');
+    }
+  },
+
   // Alias for showToast
   toast(message, type = 'info') {
     return this.showToast(message, type);

src/web/routes/system-routes.ts

@@ -6,10 +6,11 @@
 
 import { FastifyInstance } from 'fastify';
 import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
 import { existsSync, mkdirSync, readdirSync } from 'node:fs';
 import fs from 'node:fs/promises';
 import { totalmem, freemem, loadavg, cpus } from 'node:os';
-import { execSync } from 'node:child_process';
+import { execSync, spawn } from 'node:child_process';
 import { randomBytes } from 'node:crypto';
 import { dataPath } from '../../config/instance.js';
 import { ApiErrorCode, createErrorResponse, getErrorMessage, type NiceConfig } from '../../types.js';
@@ -240,6 +241,36 @@ export function registerSystemRoutes(
     return { success: true };
   });
 
+  // ═══════════════════════════════════════════════════════════════
+  // Multi-monitor: span Codeman across all displays
+  // ═══════════════════════════════════════════════════════════════
+
+  // Spawn scripts/span-codeman.sh, which opens a fresh, maximized browser --app
+  // window sized to the union of all displays — so in-page floating session
+  // panels can be dragged across the physical monitor seam. macOS only; needs
+  // the one-time "Displays have separate Spaces" OFF prerequisite (see script).
+  app.post('/api/system/span-displays', async (req, reply) => {
+    // Resolve the bundled launcher relative to this module (works from src/ and dist/).
+    const scriptPath = join(dirname(fileURLToPath(import.meta.url)), '../../../scripts/span-codeman.sh');
+    if (!existsSync(scriptPath)) {
+      return reply.code(500).send(createErrorResponse(ApiErrorCode.INTERNAL_ERROR, 'span-codeman.sh not found'));
+    }
+    // Point the spanning window at THIS server. Pin the host to localhost (same
+    // machine) and take only a digits-only port from the Host header so nothing
+    // attacker-controllable reaches the launched browser.
+    const hostPort = String(req.headers.host ?? '').split(':')[1] ?? '';
+    const port = /^\d+$/.test(hostPort) ? hostPort : '5000';
+    const url = `http://localhost:${port}`;
+    try {
+      const child = spawn('bash', [scriptPath, url], { detached: true, stdio: 'ignore' });
+      child.on('error', (err) => app.log.error({ err }, 'span-displays launch failed'));
+      child.unref();
+      return { success: true, url };
+    } catch (err) {
+      return reply.code(500).send(createErrorResponse(ApiErrorCode.INTERNAL_ERROR, getErrorMessage(err)));
+    }
+  });
+
   // ═══════════════════════════════════════════════════════════════
   // CLI Integrations (OpenCode)
   // ═══════════════════════════════════════════════════════════════