fix(ui): pin the OPERATIONS tab bar while panel content scrolls (#31)

The TabsList is now position:sticky top:0 with a z-index above the panel
content. In wide mode it pins to the top of the OPERATIONS scroll area
(#21's center-pane scroller); in narrow mode, where the body scrolls and
there is no inner scroll container, it pins to the viewport. The existing
opaque background keeps scrolled content from showing through.

e2e/smoke.spec.ts

@@ -82,6 +82,36 @@ test('active tab survives a page reload (#27)', async ({ page }) => {
   await expect(page.getByRole('tab', { name: 'CREW' })).toHaveAttribute('aria-selected', 'true');
 });
 
+test('tab bar stays pinned while panel content scrolls (#31)', async ({ page }) => {
+  await page.goto('/');
+  // Shrink the viewport so the center pane's content overflows its scroll area regardless of save state.
+  await page.setViewportSize({ width: 1280, height: 560 });
+
+  // WORLD MAP is always tall enough to overflow (fixed-height chart) — deterministic across retries.
+  await page.getByRole('tab', { name: 'WORLD MAP' }).click();
+
+  const tablist = page.getByRole('tablist');
+  await expect(tablist).toBeVisible();
+  const before = await tablist.boundingBox();
+  expect(before).not.toBeNull();
+
+  // Scroll the nearest scroll container above the tab list all the way down.
+  const scrolled = await page.evaluate(() => {
+    let el = document.querySelector('[role="tablist"]')?.parentElement ?? null;
+    while (el && !['auto', 'scroll'].includes(getComputedStyle(el).overflowY)) el = el.parentElement;
+    if (!el) throw new Error('no scroll container above the tab list');
+    el.scrollTop = el.scrollHeight;
+    return el.scrollTop;
+  });
+  expect(scrolled).toBeGreaterThan(50); // content really overflowed — guards against a vacuous pass
+
+  await expect(tablist).toBeVisible();
+  const after = await tablist.boundingBox();
+  expect(after).not.toBeNull();
+  // Pinned: the tab bar's viewport position is unchanged even though the content scrolled away (#31).
+  expect(Math.abs(after!.y - before!.y)).toBeLessThan(2);
+});
+
 test('world map shows the generated starting system and its neighbours', async ({ page }) => {
   const world = (await (await page.request.get('/api/world')).json()) as WorldState;