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.

components/homework/HomeworkCheckResult.tsx

@@ -42,7 +42,7 @@ export function HomeworkCheckResult({
         <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">
           {t("homework.check.verdict")}
         </p>
-        <p className="text-[15px] leading-relaxed text-[var(--text)]">{record.verdict}</p>
+        <p className="whitespace-pre-wrap break-words text-[15px] leading-relaxed text-[var(--text)]">{record.verdict}</p>
       </section>
 
       {record.correct.length > 0 ? (
@@ -66,10 +66,10 @@ export function HomeworkCheckResult({
           <ul className="mt-2 space-y-3">
             {record.wrong.map((entry, index) => (
               <li key={index} className="space-y-0.5 text-[14px] leading-relaxed text-rose-900 dark:text-rose-100">
-                <p className="font-semibold">{entry.issue}</p>
-                <p>{entry.why}</p>
+                <p className="whitespace-pre-wrap break-words font-semibold">{entry.issue}</p>
+                <p className="whitespace-pre-wrap break-words">{entry.why}</p>
                 {entry.fix ? (
-                  <p>
+                  <p className="whitespace-pre-wrap break-words">
                     <span className="font-bold">{t("homework.check.fix")}: </span>
                     {entry.fix}
                   </p>