fix(run): keep the tmux pane exactly as wide as xterm

Scrolling a lane console repainted history with stray characters glued to
the left edge of blank lines. Reproduced against a real tmux attach: a pane
WIDER than xterm makes every over-long row autowrap its tail onto the next
row's left margin, and tmux — believing that row unchanged — never repaints
over it. One column of difference leaves one stray character per wrapped
line, two columns leave two; matched widths render clean.

TerminalView only reported its size on WS open and on the container's
ResizeObserver, so nothing corrected:
- cols moving with no container resize (webfont load, page zoom, scrollbar)
- tmux's `window-size latest` letting a second attach client (another tab,
  `ccam lanes shell`) resize the window out from under this one

Size is now reported from xterm's own onResize, re-asserted when the
terminal regains focus, and never reported as zero — the console collapses
with CSS, and fitting a 0x0 container would resize the real pane to nothing.

client 447/447, tsc clean.

client/src/components/run/TerminalView.tsx

@@ -59,6 +59,25 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
 
     const decoder = new TextDecoder();
     let ws: WebSocket | null = null;
+
+    /**
+     * Tell the server this terminal's size. tmux draws the pane at whatever
+     * width it last heard, and a pane WIDER than xterm is silently corrupting:
+     * every over-long row autowraps its tail onto the next row's left margin,
+     * and tmux — believing that row unchanged — never repaints over it. The
+     * result is stray characters glued to the left edge of blank lines, which
+     * only becomes visible once a scroll repaints history. Reproduced with a
+     * one-column difference; two columns leaves two characters per line.
+     *
+     * A zero-size fit is never reported: the console collapses with CSS
+     * (`display:none` leaves the container 0×0), and fitting to that would
+     * resize the real tmux pane down to nothing.
+     */
+    const sendSize = () => {
+      if (!ws || ws.readyState !== WebSocket.OPEN) return;
+      if (term.cols < 2 || term.rows < 2) return;
+      ws.send(JSON.stringify({ type: "resize", cols: term.cols, rows: term.rows }));
+    };
     let retryTimer: ReturnType<typeof setTimeout> | null = null;
     let ended = false; // server said the pane exited — nothing left to attach to
     let disposed = false;
@@ -72,7 +91,7 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
       sock.binaryType = "arraybuffer";
 
       sock.onopen = () => {
-        sock.send(JSON.stringify({ type: "resize", cols: term.cols, rows: term.rows }));
+        sendSize();
       };
       sock.onmessage = (event) => {
         const isArrayBuffer = Object.prototype.toString.call(event.data) === "[object ArrayBuffer]";
@@ -113,11 +132,23 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
       if (ws && ws.readyState === WebSocket.OPEN) ws.send(data);
     });
 
+    // xterm's own size change is the event worth reporting, not the
+    // container's: cols also move when the webfont finishes loading or the
+    // page zooms, neither of which resizes the element ResizeObserver watches.
+    const resizeDisposable = term.onResize(sendSize);
+
+    // Re-assert on focus. Several attach clients can share one tmux session
+    // (a second tab, another lane pane), and tmux's `window-size latest` sizes
+    // the window to whichever client was last active — resizing this pane out
+    // from under a terminal that is never told. Coming back to it says so.
+    const reassert = () => sendSize();
+    term.textarea?.addEventListener("focus", reassert);
+
     const resizeObserver = new ResizeObserver(() => {
-      fit.fit();
-      if (ws && ws.readyState === WebSocket.OPEN) {
-        ws.send(JSON.stringify({ type: "resize", cols: term.cols, rows: term.rows }));
-      }
+      // Never fit a collapsed console — see sendSize.
+      const el = containerRef.current;
+      if (!el || !el.clientWidth || !el.clientHeight) return;
+      fit.fit(); // a real change fires onResize, which reports it
     });
     if (containerRef.current) resizeObserver.observe(containerRef.current);
 
@@ -125,6 +156,8 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
       disposed = true;
       if (retryTimer) clearTimeout(retryTimer);
       resizeObserver.disconnect();
+      term.textarea?.removeEventListener("focus", reassert);
+      resizeDisposable.dispose();
       dataDisposable.dispose();
       ws?.close();
       term.dispose();