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/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)
+}