fix(webui): route the terminal's font wait through the browser platform

The workspace panel reached for `document` directly to wait for the fonts
before it measured the terminal's grid. The dependency rules confine that
global to the platform layer, and the platform is where the wait belongs:
a component that reaches for `document` itself is a component that can
only be placed against a browser.

The browser platform now owns the wait (`whenFontsReady`), and the panel
asks for it the way it asks for every other browser capability.

This is the baseline's own `check:webui-dependency` failure — the gate
reported these two accesses on the untouched `webui` branch.

Author: stevenjj33

packages/webui/src/client/features/workspace/ui/WorkspacePanels.tsx

@@ -525,7 +525,12 @@ export function WebuiWorkspacePanel({ state, dispatch, sessionId, workspaceDir,
     terminal.open(host);
     let disposed = false;
     let initialFitFrame: number | undefined;
-    void document.fonts.ready.then(() => {
+    // The grid is measured once the fonts have landed: a measurement taken
+    // against fallback glyphs is a measurement of the wrong grid. The wait
+    // comes from the browser platform, because a panel that reached for
+    // `document` itself would be a panel that could only be placed against a
+    // browser.
+    void dom.whenFontsReady().then(() => {
       if (disposed) return;
       initialFitFrame = requestAnimationFrame(() => {
         if (disposed) return;
@@ -570,7 +575,7 @@ export function WebuiWorkspacePanel({ state, dispatch, sessionId, workspaceDir,
     const input = terminal.onData((data) => { if (terminalOwner?.canWrite) void terminalOwner.write(terminalId, data); });
     let disposed = false;
     let initialFitFrame: number | undefined;
-    void document.fonts.ready.then(() => {
+    void dom.whenFontsReady().then(() => {
       if (disposed) return;
       initialFitFrame = requestAnimationFrame(() => {
         if (disposed) return;

packages/webui/src/client/platform/browser-dom.ts

@@ -101,6 +101,20 @@ export interface WebuiBrowserDom {
    * palette.
    */
   readonly readCssVariable: (name: string, fallback: string) => string;
+  /**
+   * Resolves once the document's fonts have loaded.
+   *
+   * A terminal measures its grid after the fonts land: a measurement taken
+   * against fallback glyphs is a measurement of the wrong grid, and the shell
+   * that follows it resizes to a size the terminal never had. The wait
+   * belongs to the platform for the same reason the resize listener does — a
+   * component that reaches for `document` directly is a component that cannot
+   * be placed against anything but a browser.
+   *
+   * A platform with no document resolves at once: there are no fonts to wait
+   * for, so the caller measures the grid it has.
+   */
+  readonly whenFontsReady: () => Promise<void>;
   /**
    * Creates a drawing surface.
    *
@@ -223,6 +237,11 @@ export function createWebuiBrowserDom(
         .trim();
       return value || fallback;
     },
+    whenFontsReady: () => {
+      const fonts = documentObject?.fonts;
+      if (!fonts) return Promise.resolve();
+      return fonts.ready.then(() => undefined);
+    },
     createCanvas: (width, height) => {
       if (!documentObject) return null;
       const canvas = documentObject.createElement("canvas");