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.

src/web/public/styles.css

@@ -621,8 +621,9 @@ body {
    and only the rows beside the toolbar stop short of it. A flex line cannot
    be shortened on its own, so the strip lays its tabs out inline, around a
    float the size of the toolbar's box (measured into --tab-corner-width and
-   --tab-corner-height). The strip scrolls, so it contains the float and is
-   never shorter than the toolbar it now holds up. Inline tabs space their
+   --tab-corner-height). The strip is a scroll container, so it contains the
+   float and is never shorter than the toolbar it now holds up; a strip that
+   would actually scroll keeps the plain wrap instead. Inline tabs space their
    rows with a bottom margin (the old row-gap, lineage's included); the strip
    carries no font size of its own, so the whitespace between tabs is 0 wide. */
 .header:has(> .session-tabs-host > .session-tabs.tabs-corner) > .header-right {