Terminal: a shell on the agent chat's computer, before the first message too

The Terminal panel opens a shell on the device an agent chat works on, in its
folder: for a new agent chat as soon as the device and folder are chosen (the
server's /api/v2/devices/{device}/terminal/ws?directory=, which the chat's first
message takes over -- the same socket carries on), and for an open agent chat
from the header (/api/v2/chats/{chat}/terminal/ws, the web page's shell).
Offered where GET /devices says the device opens a terminal.

The socket is Rust's (src-tauri/src/terminal.rs, tungstenite on the rustls
ureq already brings): the bearer token goes in the handshake and never reaches
the page, which gets the frames as `terminal` events and sends keys, sizes and
close through commands. 4401 signs the app out as a 401 does.

xterm and its fit addon are the server's vendored build, pinned through
fetch-vendor.sh like CodeMirror and loaded on the first open. Send to chat puts
the selection, the last marked command and its output, or the screen into the
composer, never sent. The terminal and the activity panel take turns in the
panel column. Continue in a terminal stays in the chat's menu.

tests/ui: seven B3 cases with a mocked socket, at 1280 and 390 px.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ui/js/bridge.js

@@ -66,6 +66,61 @@ export async function follow(path, { onFrame, onEnd }) {
   }
 }
 
+// --- The terminal panel's socket -----------------------------------------------------------------
+//
+// Opened by Rust (`terminal_open`), which holds the token and the socket; the page gets the frames
+// as `terminal` events -- `output` (the shell's bytes, base64), `control` (the server's JSON), a last
+// `end` -- and sends keys, sizes and "close" through commands. As with a followed reply, a frame
+// can come before `terminal_open` has answered with its id: those wait.
+
+const terminals = new Map()
+const earlyTerminal = new Map()
+listen("terminal", (e) => {
+  const { id, kind, data } = e.payload
+  const f = terminals.get(id)
+  if (f) deliverTerminal(f, id, kind, data)
+  else {
+    if (!earlyTerminal.has(id)) earlyTerminal.set(id, [])
+    earlyTerminal.get(id).push([kind, data])
+  }
+})
+
+function deliverTerminal(f, id, kind, data) {
+  if (kind === "end") {
+    terminals.delete(id)
+    f.onEnd(data || {})
+  } else if (kind === "output") f.onOutput(data)
+  else if (kind === "control") f.onControl(data)
+}
+
+/**
+ * Open a shell: `target` is `{ chat }` or `{ device, directory }`; `onOutput(base64)`,
+ * `onControl(frame)`, and `onEnd({ code, reason, status, error })` once. The handle sends keys
+ * (`input`), a size (`resize`), and lets go (`close`; onEnd still comes).
+ */
+export async function openTerminal(target, cols, rows, { onOutput, onControl, onEnd }) {
+  const id = await invoke("terminal_open", {
+    chat: target.chat ?? null,
+    device: target.device ?? null,
+    directory: target.directory ?? null,
+    cols,
+    rows,
+  })
+  const f = { onOutput, onControl, onEnd }
+  terminals.set(id, f)
+  for (const [kind, data] of earlyTerminal.get(id) || []) {
+    if (!terminals.has(id)) break
+    deliverTerminal(f, id, kind, data)
+  }
+  earlyTerminal.delete(id)
+  return {
+    id,
+    input: (data) => invoke("terminal_input", { id, data }),
+    resize: (c, r) => invoke("terminal_resize", { id, cols: c, rows: r }),
+    close: () => invoke("terminal_close", { id }),
+  }
+}
+
 /** A web link, in the system's browser. */
 export function openLink(url) {
   return invoke("open_external", { url }).catch(() => {})