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/chat/ChatScreen.tsx

@@ -681,8 +681,18 @@ export function ChatScreen() {
           online={online}
           scopeLabel={scopeText}
           modelLabel={activeModel || t("chat.modelTitle")}
-          onOpenScope={() => setScopeOpen(true)}
-          onOpenModel={() => setModelOpen(true)}
+          scopeOpen={scopeOpen}
+          modelOpen={modelOpen}
+          // The chips toggle their own popover (and open one at a time), so a
+          // second tap closes instead of forcing a pick.
+          onOpenScope={() => {
+            setScopeOpen((open) => !open);
+            setModelOpen(false);
+          }}
+          onOpenModel={() => {
+            setModelOpen((open) => !open);
+            setScopeOpen(false);
+          }}
           onAttachFile={(file) => void handleAttach(file)}
           attachBusy={attachBusy}
           attachments={attachments}