refactor(ui): fold the 36px header/search buttons into IconButton

Completes the IconButton extraction by adding `size` and `radius` props and
converting the two remaining outliers that were left out of the first pass:
the desktop header logout button (+layout) and the home search submit button
(+page), both 36px with the medium radius.

IconButton now drives every icon button in the app (except profile/history's,
which the shared-HistoryList change removes separately). Defaults stay 32px /
small radius, so the four already-converted sites are untouched. Rendered
size, radius, and variant match the old markup — no behaviour change, no
version bump.

This also sets up applying the --tap-min touch floor in one place once the
touch-target change lands.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/IconButton.svelte.test.ts

@@ -37,6 +37,23 @@ describe('IconButton', () => {
         expect(c2.querySelector('button.icon-btn.danger')).toBeTruthy();
     });
 
+    it('defaults to a 32px square with the small radius', () => {
+        const { container } = render(IconButton, { props: { children: icon } });
+        const btn = container.querySelector('button.icon-btn') as HTMLButtonElement;
+        expect(btn.classList.contains('radius-sm')).toBe(true);
+        expect(btn.style.getPropertyValue('--ib-size')).toBe('32px');
+    });
+
+    it('honours an explicit size and radius (the larger header/search buttons)', () => {
+        const { container } = render(IconButton, {
+            props: { size: 36, radius: 'md', children: icon }
+        });
+        const btn = container.querySelector('button.icon-btn') as HTMLButtonElement;
+        expect(btn.style.getPropertyValue('--ib-size')).toBe('36px');
+        expect(btn.classList.contains('radius-md')).toBe(true);
+        expect(btn.classList.contains('radius-sm')).toBe(false);
+    });
+
     it('forwards onclick', async () => {
         const onclick = vi.fn();
         render(IconButton, { props: { onclick, children: icon } });