feat(ui): connection status in the header; README screenshots as WebP

- Desktop shows "offline" with a red dot under the user name when the
  server is unreachable; phones show a green/red dot on the avatar.
  Based on the app-wide realtime connection, so a server outage is
  noticed even while the device has network (4 s grace period).
- Screenshots converted to WebP (1280 px desktop, 480 px phone, q90),
  e2e/screenshots.sh converts with cwebp; new journal picture with
  round corners. Rules in AGENTS.md.
- Release v0.26.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

e2e/screenshots.mjs

@@ -196,7 +196,10 @@ const main = async () => {
   await page.getByRole('button', { name: 'Heute' }).click().catch(() => {});
   await page.mouse.click(1000, 860);
   await sleep(2500);
+  // Round corners (setting "Ecken: rund") for this picture only
+  await page.evaluate(() => document.documentElement.setAttribute('data-corners', 'round'));
   await plain(page, 'journal_desktop.png', 'dark');
+  await page.evaluate(() => document.documentElement.removeAttribute('data-corners'));
   await page.getByRole('button', { name: 'Journal', exact: true }).click(); // close it again
 
   await tab(page, 'Listen');