feat(tabs): L-shaped header strip, wrapped rows run under the toolbar

A wrapped classic tab strip stopped at the toolbar's left edge on every row,
so the space under the header chips stayed empty and long tabs took a row
each. Once the strip wraps, the toolbar now sits over the strip's top-right
corner and the strip lays its tabs out inline around a float sized to the
toolbar's box (_syncTabStripCorner), so only the rows beside the toolbar stop
short of it and every later row runs the header's full width.

- Wrap is still measured with the toolbar in flow (the corner is dropped
  before the measure), so the single-row strip is unchanged.
- A ResizeObserver on the toolbar re-decides wrap + corner and redraws the
  lineage routes when a reading or the divider changes its size.
- Falls back to the plain strip when the first tab cannot fit beside the
  toolbar (it would otherwise leave an empty band).
- Classic arrangement, desktop, header host only; state/case/ledger, the
  sidebar and the tab rail keep their own layouts.

src/web/public/styles.css

@@ -616,6 +616,44 @@ body {
   padding-left: var(--lineage-spine-channel);
 }
 
+/* L-shaped strip (`_syncTabStripCorner()`, app.js): a wrapped classic strip
+   runs the header's full width, the toolbar sits over its top-right corner,
+   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
+   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 {
+  position: absolute;
+  top: 0.35rem;
+  right: 0.75rem;
+}
+
+.session-tabs-host > .session-tabs.tabs-corner {
+  display: block;
+  font-size: 0;
+  padding-bottom: 0;
+}
+
+.session-tabs-host > .session-tabs.tabs-corner::before {
+  content: '';
+  float: right;
+  width: var(--tab-corner-width, 0px);
+  height: var(--tab-corner-height, 0px);
+}
+
+.session-tabs-host > .session-tabs.tabs-corner > .session-tab {
+  display: inline-flex;
+  vertical-align: top;
+  margin: 0 2px var(--lineage-row-gap, 4px) 0;
+}
+
+.session-tabs-host > .session-tabs.lineage-tree.tabs-corner {
+  padding-bottom: calc(var(--lineage-bottom-room) - var(--lineage-row-gap));
+}
+
 .session-tabs::-webkit-scrollbar {
   width: 4px;
   height: 0;