feat(v0.4): multi-device discovery + share link (frontend)

- tabs.ts: on open, syncLiveSessions() fetches /live-sessions and adds a tab for
  each host session not already open → any device sees & mirrors everything
  running on the host. Empty storage + no live sessions → one fresh tab.
  activeSessionId() + openSession(id) helpers.
- share.ts: 🔗 toolbar button → QR + <origin>/?join=<id> for the active session.
- main.ts: ?join=<id> on load opens/focuses that shared session, then strips the
  param. Mount share button (toolbar: 🔍 ⚙ ▦ 🕘 ⌨ 🔗 📱).
- style.css: #sharemodal reuses the QR card/backdrop.

Verified in-browser: fresh device auto-discovered the host session and replayed
its scrollback; two concurrent clients (browser + ws) both received live output
(clientCount=2); share modal shows the join URL/QR.

public/main.ts

@@ -19,6 +19,7 @@ import { mountSettings, loadSettings, type Settings } from './settings.js'
 import { mountDashboard } from './dashboard.js'
 import { mountHistory } from './history.js'
 import { mountShortcuts } from './shortcuts.js'
+import { mountShareSession } from './share.js'
 
 const paneHost = document.getElementById('term')
 const tabs = document.getElementById('tabs')
@@ -29,6 +30,14 @@ if (!toolbar) throw new Error('#toolbar element not found in DOM')
 
 const app = new TabApp(paneHost, tabs)
 
+// v0.4: a share link (?join=<id>) opens/focuses that shared session, then we
+// strip the param so a refresh doesn't keep re-opening it.
+const joinId = new URLSearchParams(location.search).get('join')
+if (joinId) {
+  app.openSession(joinId)
+  history.replaceState(null, '', location.pathname)
+}
+
 // Apply saved theme/font (M3) to the restored tabs.
 let settings: Settings = loadSettings()
 app.applySettings(settings)
@@ -57,6 +66,7 @@ mountHistory(toolbar, {
   resume: (cwd, id) => app.newTabForResume(cwd, id),
 })
 mountShortcuts(toolbar)
+mountShareSession(toolbar, () => app.activeSessionId())
 mountQrConnect(toolbar)
 
 // PWA: register the service worker (installable + offline shell, M4).

public/share.ts

@@ -0,0 +1,72 @@
+/**
+ * public/share.ts — "share this session" (v0.4 multi-device mirror).
+ *
+ * A 🔗 toolbar button shows a QR + URL of the form `<origin>/?join=<sessionId>`
+ * for the ACTIVE tab. Opening that link on another device on the same network
+ * joins the same live terminal (mirror: both see output, either can type).
+ * Reuses the .qr-* card styles.
+ */
+
+import QRCode from 'qrcode'
+
+export function mountShareSession(toolbar: HTMLElement, getActiveId: () => string | null): void {
+  const modal = document.createElement('div')
+  modal.id = 'sharemodal'
+  modal.className = 'qrmodal'
+  modal.style.display = 'none'
+
+  const card = document.createElement('div')
+  card.className = 'qr-card'
+  const title = document.createElement('div')
+  title.className = 'qr-title'
+  title.textContent = 'Share this session'
+  const canvas = document.createElement('canvas')
+  const urlEl = document.createElement('div')
+  urlEl.className = 'qr-url'
+  const note = document.createElement('div')
+  note.className = 'qr-note'
+  const close = document.createElement('button')
+  close.className = 'qr-close'
+  close.textContent = 'Close'
+  card.append(title, canvas, urlEl, note, close)
+  modal.appendChild(card)
+  document.body.appendChild(modal)
+
+  const hide = (): void => {
+    modal.style.display = 'none'
+  }
+  modal.addEventListener('click', (e) => {
+    if (e.target === modal) hide()
+  })
+  close.addEventListener('click', hide)
+
+  const open = (): void => {
+    const id = getActiveId()
+    if (!id) {
+      // The active tab hasn't attached yet — no id to share.
+      urlEl.textContent = ''
+      note.textContent = 'This tab is still connecting — try again in a second.'
+      const ctx = canvas.getContext('2d')
+      if (ctx) ctx.clearRect(0, 0, canvas.width, canvas.height)
+      modal.style.display = 'flex'
+      return
+    }
+    const url = `${location.origin}/?join=${id}`
+    urlEl.textContent = url
+    void QRCode.toCanvas(canvas, url, { width: 220, margin: 1 })
+    const host = location.hostname
+    note.textContent =
+      host === 'localhost' || host === '127.0.0.1'
+        ? 'Tip: open this page via your LAN IP (not localhost) so the code works on other devices.'
+        : 'Scan on a device on the same network to mirror this session.'
+    modal.style.display = 'flex'
+  }
+
+  const toggle = document.createElement('button')
+  toggle.className = 'toolbtn'
+  toggle.textContent = '🔗'
+  toggle.title = 'Share this session (mirror it on another device)'
+  toggle.setAttribute('aria-label', 'Share this session')
+  toggle.addEventListener('click', open)
+  toolbar.appendChild(toggle)
+}

public/style.css

@@ -368,8 +368,9 @@ body {
   background: var(--accent-soft);
 }
 
-/* QR modal */
-#qrmodal {
+/* QR + share modals */
+#qrmodal,
+#sharemodal {
   position: fixed;
   inset: 0;
   z-index: 1200;

public/tabs.ts

@@ -133,7 +133,9 @@ export class TabApp {
       } catch {
         legacy = null
       }
-      stored = [{ id: legacy, title: null }]
+      // v0.2.x single-session migration only; otherwise stay empty and let
+      // syncLiveSessions() decide (host sessions, or a fresh tab as fallback).
+      if (legacy) stored = [{ id: legacy, title: null }]
     }
 
     for (const s of stored) this.addEntry(s.id, s.title)
@@ -146,7 +148,71 @@ export class TabApp {
     }
     if (!Number.isInteger(active) || active < 0 || active >= this.tabs.length) active = 0
     this.rebuild()
-    this.activate(active)
+    if (this.tabs.length > 0) this.activate(active)
+
+    // v0.4: discover the host's running sessions and show them as tabs too.
+    void this.syncLiveSessions()
+  }
+
+  /**
+   * v0.4 multi-device: fetch the host's currently-running sessions and add a tab
+   * for each one not already open, so opening the app on any device shows (and
+   * joins/mirrors) everything running on the host. Falls back to one fresh tab
+   * when nothing is stored and nothing is live.
+   */
+  private async syncLiveSessions(): Promise<void> {
+    let live: Array<{ id: string }> = []
+    try {
+      const res = await fetch('/live-sessions')
+      const data: unknown = await res.json()
+      if (Array.isArray(data)) live = data as Array<{ id: string }>
+    } catch {
+      live = []
+    }
+
+    const open = new Set(
+      this.tabs.map((t) => t.session.id).filter((x): x is string => typeof x === 'string'),
+    )
+    let added = false
+    for (const info of live) {
+      if (typeof info.id === 'string' && !open.has(info.id)) {
+        this.addEntry(info.id, null)
+        added = true
+      }
+    }
+    // Nothing restored and nothing live → start one fresh session.
+    if (this.tabs.length === 0) {
+      this.addEntry(null, null)
+      added = true
+    }
+    if (!added) return
+
+    this.rebuild()
+    const idx =
+      this.activeIndex >= 0 && this.activeIndex < this.tabs.length ? this.activeIndex : 0
+    this.activeIndex = idx
+    this.tabs.forEach((t, i) => (i === idx ? t.session.show() : t.session.hide()))
+    this.tabs.forEach((t) => this.refreshTab(t))
+    this.updateApprovalBar()
+    this.persist()
+  }
+
+  /** The active tab's server session id (null until it has attached). */
+  activeSessionId(): string | null {
+    return this.tabs[this.activeIndex]?.session.id ?? null
+  }
+
+  /** Focus the tab for `sessionId`, or open one that joins it (share-link target). */
+  openSession(sessionId: string): void {
+    const existing = this.tabs.findIndex((t) => t.session.id === sessionId)
+    if (existing >= 0) {
+      this.activate(existing)
+      return
+    }
+    this.addEntry(sessionId, null)
+    this.persist()
+    this.rebuild()
+    this.activate(this.tabs.length - 1)
   }
 
   /* ── tab lifecycle ───────────────────────────────────────────── */