fix: keep the host readings' labels under header density, hide ringless ones at Compact tiny (release review)

#689's density steps hide every `.header-system-stats .stat-label` once the
toolbar is narrowed, and #692's readings (GPU, VRAM, GPU MHz, CPU MHz) have
nothing else naming them: at tight, dense and tiny the header showed "40%"
beside CPU's "12%", "3.2G" beside MEM's "7.9G", and two bare "1800 MHz" /
"4200 MHz" with no way to tell the GPU clock from the CPU one. At Compact +
tiny, where values are hidden too, the two clocks (no ring) were left as
0-px items that still took an 8 px flex gap each. Neither PR shows it alone.

- The four host items carry `stat-item--host`, excluded from the density
  label hide, so they keep their labels at every step.
- `stat-item--ringless` (static on the two clocks, toggled by
  `_updateHostSensors` on VRAM while its total, and so its ring, is unknown)
  is hidden at Compact + tiny instead of staying an empty item.

Tests: the browser test drives every density in all three styles against
the real markup and styles.css (it fails on the previous files), and the
unit test pins the VRAM ringless toggle.

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

docs/wiki/The-Dashboard.md

@@ -180,7 +180,9 @@ Hiding System Stats or Plan Usage still hides them in every style.
 
 The line between the session tabs and the toolbar is a handle: drag it right to narrow the toolbar
 (it wraps onto a second line and the usage readings shed their labels, then their provider names,
-then, in the Compact style, their percentages), left to widen it. Arrow keys resize it, and
+then, in the Compact style, their percentages), left to widen it. The GPU, VRAM and clock readings
+keep their labels, since their values look like CPU's and MEM's; at the narrowest Compact step a
+clock, which has no ring, is hidden. Arrow keys resize it, and
 double-click or Home resets it. It stops at the widest reading, the width is kept per device, and it
 is not shown on phones, in the sidebar layout, in the vertical rail or in a solo window.