fix(mobile): make the phone header tab strip read as live tabs

On a phone every inactive tab rendered transparent: grey 11px text
floating in unmarked gaps, a boxed Alt+N digit in each tab (a phone has
no Alt key), names capped at 50px so a shared `w1-` prefix was most of
what showed, and the tab that did not fit was chopped mid-word against
the connection dot. The strip looked like a row of disabled labels.

Phone block of mobile.css only:
- Every header tab is a chip, filled and bordered from the skin's
  --control-* tokens, name in --text at weight 500. Written
  `:where(.header) .session-tab` so it stays at (0,1,0): the per-colour
  left border still wins, and sidebar layout (where the list leaves the
  header) is untouched.
- The Alt+N digit is hidden in the header; inactive tabs drop their
  empty .tab-actions container, which padded the chip's right side.
- Name cap 50px -> 80px, status dot 4px -> 6px, strip gap 2px -> 6px.
- Scroll-driven edge fade: a mask on the strip whose widths follow its
  own inline scroll timeline (registered @property lengths), so the
  clipped tab dissolves into the edge. No JS; a strip that does not
  overflow gets no mask, and browsers without scroll timelines keep the
  old hard edge.

The tap-zone arithmetic comment is updated for the numberless phone
tabs and the bigger dot (the required reserve drops from 38px to 36px;
the 44px min-width stays). test/mobile-tab-strip-chips.test.ts pins the
(0,1,0) selector, the top-level @property registration and the
timeline-after-shorthand order, each of which fails silently otherwise.
test/mobile/tabs.test.ts follows the new name cap.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/mobile/tabs.test.ts

@@ -88,9 +88,10 @@ describe('Tab Navigation', () => {
       if (tabNameExists) {
         const maxWidth = await getCSSProperty(page, SELECTORS.TAB_NAME, 'max-width');
         const maxWidthPx = parseFloat(maxWidth);
-        // Should be 50px on mobile
-        expect(maxWidthPx).toBeLessThanOrEqual(60);
-        expect(maxWidthPx).toBeGreaterThan(0);
+        // 80px on phones: wide enough to get past a shared `w1-` prefix,
+        // still short enough that several tabs fit the strip.
+        expect(maxWidthPx).toBeLessThanOrEqual(96);
+        expect(maxWidthPx).toBeGreaterThanOrEqual(72);
       }
     });