One-pill study chrome, toggling chat pickers, readable homework steps

UI batch from feedback:

- The study/review/exam/homework chrome is now a single 58px pill: round
  close button, title + position over the progress bar, chips (or the exam
  timer) at the far end — everything on the centre line, so nothing crowds
  the rounded corners any more. Homework guide gets the same recipe.
- The immersive feeds carry z-[45] so they paint over the app chrome (on
  /exam the header used to cover the top of the chrome), and the page chrome
  behind an open overlay (.app-header, .segmented, .tabbar) is hidden while
  the transparent overlay is up, so it can no longer ghost through the glass.
  The guide's disclaimer moved inside the overlay with it.
- Chat: the context and model chips toggle their own popover (pressed state,
  aria-expanded), opening one closes the other, a press outside closes it —
  no more being forced to pick something to get out.
- Homework: the model's plain-text step explanations/hints/verdicts keep
  their line breaks (whitespace-pre-wrap) — a step used to collapse into one
  run-on paragraph with the whole solution inside it; the guide prompt now
  says the explanation is coaching, 2-4 sentences.
- The four glacier blobs are at 25% of their old opacity (0.045 light,
  0.1 dark).

Tests: phase-2 chrome geometry + exam layering checks (62), chat desktop
picker checks (101), the standing suites all green. README: decisions 54-56,
decision 23 updated. SW -> freezmo-v44.

app/globals.css

@@ -173,18 +173,21 @@
   --shadow-card-hover: 0 24px 60px -24px rgb(30 64 175 / 0.55), 0 2px 8px rgb(0 0 0 / 0.3);
 }
 
-/* Ice scene tokens: the fixed layered background behind everything. */
+/* Ice scene tokens: the fixed layered background behind everything.
+   The four glacier blobs are deliberately whisper-faint now: at 0.18/0.40
+   they competed with the content (the "huge circles" read as part of the
+   UI). 25% of the old values. */
 :root {
   --ice-base-top: #f4f9ff;
   --ice-base-bottom: #e6f1fb;
-  --ice-blob-opacity: 0.18;
+  --ice-blob-opacity: 0.045;
   --ice-orb-opacity: 0.34;
   --ice-grain-opacity: 0.05;
 }
 .dark {
   --ice-base-top: #070b16;
   --ice-base-bottom: #0c1426;
-  --ice-blob-opacity: 0.4;
+  --ice-blob-opacity: 0.1;
   --ice-orb-opacity: 0.5;
   --ice-grain-opacity: 0.045;
 }
@@ -217,6 +220,14 @@ body {
   background: transparent;
 }
 
+/* A full-screen overlay (study feed, review/exam session, homework guide) is
+   transparent on purpose so the fixed ice scene stays visible — which also
+   means the app chrome of the page behind it would ghost through the glass.
+   While one is open, that chrome is hidden instead. */
+body:has(.overlay-open) :is(.app-header, .segmented, .tabbar) {
+  visibility: hidden;
+}
+
 /* The key gate rides on the shared ice scene (see .ice-scene). */
 
 /* The fixed ice scene: navy/pale base, drifting glacier blobs, glossy ice