fix(file-viewer): keep the Expand label fixed while it is pressed

Merge-time fix for #659.

The Expand button swapped its title and aria-label to "Restore size"
while on, so a screen reader announced "Restore size, pressed". The
label now stays "Expand" and aria-pressed alone carries the state, the
way the MD, Lines and Wrap toggles work.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/file-preview-window.test.ts

@@ -196,16 +196,22 @@ describe('file viewer back/forward', () => {
 describe('file viewer expand', () => {
   it('toggles the window class, the pref and the button state', () => {
     const { app, winClasses, store, expand } = loadApp();
+    // As index.html ships it.
+    expand.title = 'Expand';
+    expand.attrs['aria-label'] = 'Expand';
 
     app.toggleFilePreviewExpanded();
     expect(winClasses.has('expanded')).toBe(true);
     expect(store['codeman:filePreviewExpanded']).toBe('1');
     expect(expand.attrs['aria-pressed']).toBe('true');
-    expect(expand.title).toBe('Restore size');
+    // aria-pressed carries the state; the label stays fixed, like the MD/Lines/Wrap toggles.
+    expect(expand.title).toBe('Expand');
+    expect(expand.attrs['aria-label']).toBe('Expand');
 
     app.toggleFilePreviewExpanded();
     expect(winClasses.has('expanded')).toBe(false);
     expect(expand.attrs['aria-pressed']).toBe('false');
+    expect(expand.title).toBe('Expand');
   });
 
   it('re-applies a remembered expanded state when a preview opens', async () => {