Teaching: hide-notes-until share, session length check, slides export

- A study share link can hide the owner's notes until a chosen time: the server
  sends only passages and discussion questions until then, with a banner in the
  shared view (new share_reveal column, migrated on start).
- Outline: a session length with "N of M min planned, X min over" and a marker
  on the point where the plan runs past it.
- Export > Slides: printable 16:9 slides (title, passage, outline points with
  their verse, discussion questions) for one session or all; light or dark.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/utils/present.js

@@ -41,3 +41,22 @@ export const paceText = (p) => {
   if (Math.abs(d) < 20) return 'On pace';
   return d > 0 ? `${formatClock(d)} ahead` : `${formatClock(-d)} behind`;
 };
+
+// How the planned minutes compare with the time you have. `limit` is the session length (null/0 = not set).
+// -> { state: 'unset' | 'under' | 'on' | 'over', diff (minutes, + to spare, - over), text }
+export function planStatus(total, limit) {
+  const t = Number(total) || 0;
+  const l = Number(limit) || 0;
+  if (!l) return { state: 'unset', diff: 0, text: '' };
+  const diff = l - t;
+  if (t === 0) return { state: 'under', diff, text: `No minutes planned yet for ${l} min` };
+  if (diff === 0) return { state: 'on', diff, text: `${t} of ${l} min planned, right on time` };
+  if (diff > 0) return { state: 'under', diff, text: `${t} of ${l} min planned, ${diff} min to spare` };
+  return { state: 'over', diff, text: `${t} of ${l} min planned, ${-diff} min over` };
+}
+
+// The first point that carries the plan past the session length, or null.
+export function firstOverPoint(steps, limit) {
+  const l = Number(limit) || 0;
+  return l ? steps.find((s) => s.plannedEnd > l) ?? null : null;
+}