fix(run): draw the lane console through the canvas renderer

Scrolling a console left stray characters glued to the left edge of blank
lines. Copying an affected line yields clean text, so the buffer was always
correct — only the painting drifted, which no CSS can correct.

xterm's default DOM renderer aligns each row with a single `letter-spacing`
value. That holds only while every glyph comes from one monospace font, and
TerminalView passed no fontFamily, so xterm used its default
`courier-new, courier, monospace`: Vietnamese diacritics and CJK fall
through to a fallback face whose advance width differs, glyphs shove their
neighbours along, and the row's tail spills onto the row below.

The canvas renderer draws every cell at a fixed position, so a wide glyph
can no longer displace its neighbours. Loaded best-effort — a browser that
cannot hand back a 2D context keeps the DOM renderer rather than losing the
terminal entirely.

Ruled out first, each by replaying the real byte stream into xterm.js and
diffing against `tmux capture-pane`: wrapped Vietnamese at the margin, mixed
CJK, wheel scrolling through copy-mode, and a live 131->95->131 resize all
render byte-identically at matched widths.

client 448/448, tsc clean.

client/src/components/run/TerminalView.tsx

@@ -14,6 +14,7 @@
 import { useEffect, useRef } from "react";
 import { Terminal } from "@xterm/xterm";
 import { FitAddon } from "@xterm/addon-fit";
+import { CanvasAddon } from "@xterm/addon-canvas";
 import "@xterm/xterm/css/xterm.css";
 
 interface TerminalViewProps {
@@ -29,6 +30,24 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
     const fit = new FitAddon();
     term.loadAddon(fit);
     if (containerRef.current) term.open(containerRef.current);
+
+    // Draw each cell at a fixed position instead of letting the browser lay the
+    // row out as text. xterm's default DOM renderer aligns a row with a single
+    // `letter-spacing`, which only holds while every glyph comes from one
+    // monospace font — Vietnamese diacritics and CJK fall through to a fallback
+    // face whose advance width differs, so glyphs shove their neighbours along
+    // and the row's tail spills onto the row below. The buffer is correct
+    // throughout (copying the line yields clean text); only the painting drifts,
+    // and no CSS can correct it. Best-effort: a browser that cannot give a 2D
+    // context keeps the DOM renderer rather than losing the terminal.
+    let canvas: CanvasAddon | null = null;
+    try {
+      canvas = new CanvasAddon();
+      term.loadAddon(canvas);
+    } catch {
+      canvas = null;
+    }
+
     fit.fit();
 
     // When the pane's program has NOT enabled wheel reporting, xterm falls back
@@ -160,6 +179,7 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
       resizeDisposable.dispose();
       dataDisposable.dispose();
       ws?.close();
+      canvas?.dispose();
       term.dispose();
     };
   }, [runId, wsBaseUrl]);