fix(tabs): keep the plain wrap when the strip scrolls, add a browser test

Review follow-up for the L-shaped header strip:

- _syncTabStripCorner() keeps the plain wrap when the plain strip is
  already past --tab-strip-max-height, checked before .tabs-corner goes
  on: the float scrolled with the tabs while the toolbar stayed put, so a
  scrolled strip put tabs under #headerRight, and a re-render of a
  scrolled two-rows strip sized the float from the scrolled frame. The
  "- scrollTop" in contentTop is gone (always 0 now).
- A strip just under its max-height that passes it once the float is in
  falls back to the plain wrap as well.
- test/tab-strip-corner.browser.test.ts (real Chromium, real header and
  styles.css), listed in BROWSER_TEST_GLOBS.
- src/web/CLAUDE.md Tab layouts entry describes the L-shaped classic strip.

config/test-suites.ts

@@ -27,6 +27,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/tab-focus.browser.test.ts',
   'test/session-sidebar-ux.browser.test.ts',
   'test/header-divider.browser.test.ts',
+  'test/tab-strip-corner.browser.test.ts',
   'test/header-host-sensors.browser.test.ts',
   'test/session-options-responsive.browser.test.ts',
   'test/inline-rename.test.ts',