Fix the iOS install sheet: exact Safari flow, chips, iOS blue, layout

Steps
- iOS now uses the exact flow: Safari users get 4 steps (⋯ → Share/More →
  Add to Home Screen → Add); non-Safari (CriOS/FxiOS/EdgiOS/in-app) get a
  first 'Open this page in Safari' step and 5 total. The vague 'in some
  browsers it's under…' wording is gone.

Chips and icons
- The ⋯ chip renders a single three-dot icon (icon-only pill) instead of
  the icon plus an '⋯' character (which showed two sets of dots).
- The More chip uses a chevron-down icon instead of the rounded-square.
- The accent pill uses real iOS blue: #0A84FF dark / #007AFF light, still
  the only coloured element.
- Steps and chips share one 16px type scale; chip labels are medium weight.

Typography and layout
- Numbers are small muted circles in a fixed 24px column, aligned with the
  first text line via matching line-height.
- Removed trailing periods after chips; steps tightened.
- Bottom padding is calc(24px + env(safe-area-inset-bottom)) and a
  secondary 'Maybe later' button sits below the steps.
- max-height 90dvh with inner scrolling; 36×4px low-contrast handle with
  real swipe-down-to-close (momentum off, spring back).
- App card shows the hostname only (no port), truncated.

Themes
- Dark (default): layered surfaces #1c1c1e sheet / #2c2c2e card / #3a3a3c
  chips with 8% white borders; subtle logo tile instead of solid white;
  backdrop is 50% black with 4px blur.
- Light only when the app theme was explicitly set to light, ignoring the
  system preference; borders #e0e0e0, chip fills #f2f2f2.

Also fixes a crash the test pressure exposed: a 429 (rate-limited)
/api/health response crashed Settings at health.vision.available.
fetchHealth now rejects non-OK/malformed responses so the dialog shows its
error banner instead of the app error screen.

Install-sheet suite now 64 checks; pwa/final-ux/security/contrast green.

app/globals.css

@@ -113,6 +113,11 @@
   --field: #f3f3f4;
   --field-border: #e2e2e4;
   --shadow-gate: 0 28px 70px -36px rgb(13 13 13 / 0.3), 0 1px 2px rgb(13 13 13 / 0.05);
+  /* Install sheet: light variant (dark is the default, see .dark). */
+  --install-sheet: #ffffff;
+  --install-card: #f7f7f8;
+  --install-chip: #f2f2f2;
+  --install-border: #e0e0e0;
   --shadow-card: 0 1px 2px rgb(13 13 13 / 0.06), 0 1px 3px rgb(13 13 13 / 0.04);
   --shadow-card-hover: 0 10px 28px -14px rgb(13 13 13 / 0.22), 0 1px 2px rgb(13 13 13 / 0.06);
 }
@@ -145,6 +150,11 @@
   --field: #242425;
   --field-border: rgb(255 255 255 / 0.1);
   --shadow-gate: 0 30px 80px -40px rgb(0 0 0 / 0.85), 0 1px 2px rgb(0 0 0 / 0.4);
+  /* Install sheet: layered dark surfaces (default theme for the sheet). */
+  --install-sheet: #1c1c1e;
+  --install-card: #2c2c2e;
+  --install-chip: #3a3a3c;
+  --install-border: rgb(255 255 255 / 0.08);
   --shadow-card: 0 1px 2px rgb(0 0 0 / 0.25);
   --shadow-card-hover: 0 10px 28px -16px rgb(0 0 0 / 0.55), 0 1px 2px rgb(0 0 0 / 0.3);
 }