fix: apply the review items for #689 at merge

Clamp the stored toolbar width to MIN_WIDTH in apply() and step the arrow keys from the width on screen, so a drag to the window edge or a held ArrowRight no longer stores a negative width or leaves an ArrowLeft dead zone (browser test added)
Drop Escape as a reset key on the handle: the global capture-phase Escape handler already closes every panel on it (static pin added)
Say in the changeset, the wiki and the densityFor comment that the percentages go at the narrowest step in the Compact style only, and drop Escape from the changeset

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

test/header-divider.test.ts

@@ -69,6 +69,8 @@ describe('header divider', () => {
     expect(js).toContain('localStorage');
     expect(js).not.toContain('/api/settings');
     expect(js).toMatch(/key === 'Home'/);
+    // Escape belongs to the global close-panels handler (app.js), so the handle must not claim it too.
+    expect(js).not.toMatch(/'Escape'/);
     expect(js).toContain("addEventListener('dblclick'");
   });
 });