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>

src-tauri/src/lib.rs

@@ -22,6 +22,7 @@ mod remote;
 mod sealed;
 mod server;
 mod settings;
+mod terminal;
 mod watch;
 
 use std::collections::HashMap;
@@ -115,6 +116,8 @@ struct App {
     signing_in: Mutex<Option<Arc<AtomicBool>>>,
     streams: Mutex<HashMap<u64, Arc<AtomicBool>>>,
     next_stream: AtomicU64,
+    /// The terminal panel's sockets (`terminal.rs`), by the page's id for each.
+    terminals: terminal::Sessions,
     sealed: sealed::Pages,
     watch: watch::Watch,
     /// The files last dropped on the window: what `attach_dropped` may send, and nothing else.
@@ -271,6 +274,7 @@ async fn set_server(handle: AppHandle, address: String) -> Result<Status, String
     if before.as_deref() != Some(d.base.as_str()) {
         app.agent.stop();
         app.watch.reset();
+        app.terminals.close_all();
     }
     *app.account.lock().unwrap() = server::load_account(&app.config_dir, &d.base);
     *app.server.lock().unwrap() = Some(d);
@@ -380,6 +384,7 @@ fn sign_out(handle: AppHandle) {
     let account = app.account.lock().unwrap().take();
     server::forget_account(&app.config_dir);
     app.watch.reset();
+    app.terminals.close_all();
     if let Some(a) = account {
         std::thread::spawn(move || server::sign_out(&a.server, &a.token));
     }
@@ -391,6 +396,7 @@ fn token_refused(handle: &AppHandle) {
     let app = handle.state::<App>();
     if app.account.lock().unwrap().take().is_some() {
         server::forget_account(&app.config_dir);
+        app.terminals.close_all();
         announce(handle);
     }
 }
@@ -511,6 +517,69 @@ fn stream_close(app: tauri::State<'_, App>, id: u64) {
     }
 }
 
+// --- The terminal panel ------------------------------------------------------------------------
+
+/// Open a shell on a linked device, as the terminal panel asks: an agent chat's (`chat`), or the
+/// one before the chat exists (`device` and `directory`). The socket is opened here, with the
+/// app's token in its handshake; the page gets its frames as `terminal` events with this id --
+/// `output` (base64), `control` (the server's JSON), and a last `end`.
+#[tauri::command]
+fn terminal_open(
+    handle: AppHandle,
+    chat: Option<String>,
+    device: Option<String>,
+    directory: Option<String>,
+    cols: u16,
+    rows: u16,
+) -> Result<u64, ApiError> {
+    let app = handle.state::<App>();
+    let Some((app_url, token)) = app.door() else {
+        return Err(ApiError::new(401, "Not signed in."));
+    };
+    let target = match (chat, device) {
+        (Some(chat), None) => terminal::Target::Chat(chat),
+        (None, Some(device)) => terminal::Target::Device {
+            device,
+            directory: directory.unwrap_or_default(),
+        },
+        _ => return Err(ApiError::new(400, "A chat or a device, not both.")),
+    };
+    let url = terminal::url(&app_url, &target, cols, rows)?;
+    let h = handle.clone();
+    Ok(app.terminals.open(url, token, move |down| {
+        if down.kind == "end" {
+            h.state::<App>().terminals.forget(down.id);
+            let refused = down.data.get("code").and_then(Value::as_u64)
+                == Some(u64::from(terminal::CLOSE_UNAUTHORIZED))
+                || down.data.get("status").and_then(Value::as_u64) == Some(401);
+            if refused {
+                token_refused(&h);
+            }
+        }
+        let _ = h.emit("terminal", down);
+    }))
+}
+
+/// Keys typed (or pasted) into a terminal, as text: sent to the shell as its bytes.
+#[tauri::command]
+fn terminal_input(app: tauri::State<'_, App>, id: u64, data: String) -> bool {
+    app.terminals
+        .send(id, terminal::Up::Keys(data.into_bytes()))
+}
+
+/// The terminal's size changed.
+#[tauri::command]
+fn terminal_resize(app: tauri::State<'_, App>, id: u64, cols: u16, rows: u16) -> bool {
+    app.terminals.send(id, terminal::Up::Resize(cols, rows))
+}
+
+/// The page let go of a terminal: its socket is closed (the shell on the device lives on, as
+/// the server keeps it).
+#[tauri::command]
+fn terminal_close(app: tauri::State<'_, App>, id: u64) {
+    app.terminals.close(id);
+}
+
 // --- Attachments ----------------------------------------------------------------------------------
 
 /// What became of one file sent as an attachment: the server's attachment, or why not. `reason`
@@ -1560,6 +1629,10 @@ pub fn run() {
             terminal_word,
             stream_open,
             stream_close,
+            terminal_open,
+            terminal_input,
+            terminal_resize,
+            terminal_close,
             local_list,
             local_read,
             sealed_open,
@@ -1621,6 +1694,7 @@ pub fn run() {
                 signing_in: Mutex::new(None),
                 streams: Mutex::new(HashMap::new()),
                 next_stream: AtomicU64::new(0),
+                terminals: terminal::Sessions::default(),
                 sealed: sealed::Pages::default(),
                 watch: watch::Watch::default(),
                 dropped: Mutex::new(Vec::new()),