Bigger decks: pick up to 30 activities, built in batches

The create wizard stopped at 15 activities because the API, the prompt and
the salvage path all capped at 18: one reply must be one valid JSON object,
and past ~18 activities models truncate or drop cards. Raising the cap alone
would have shipped decks that came back short.

- The count picker offers 8 / 10 / 12 / 15 / 20 / 30; a deck's "add more" row
  gained +15. The help line says bigger decks take longer.
- /api/generate accepts up to 30 (FREEZMO_DEFAULT_ACTIVITY_COUNT now clamps
  to 30 as well) and sizes its overall deadline from the number of model
  calls instead of a fixed two.
- generateActivities splits anything above 18 evenly across calls (30 → 15 +
  15, 25 → 13 + 12) and merges them into one deck. The first call keeps the
  "add to this deck" contract; later calls are told the title, the types
  already used and the topics already covered. A normalised content signature
  drops a card that repeats an earlier one, so the same idea never ships
  twice. requested/generated still travel in the stream meta.
- phase1-test: a stub Ollama answers two 15-card batches (the second repeats
  one card on purpose) → two calls, the even split, the "already covered"
  prompt, 29 merged cards, the duplicate shipped once, title from the first
  reply, and count 31 answered 400 (59 checks).
- wizard-test: the picker offers 30, the wizard sends count: 30, and the
  30-card deck is stored and opened (99 checks).

Version 0.8.4, SW v106.

lib/prompts.ts

@@ -138,8 +138,9 @@ export type GenerationRequest = {
   count: number;
   /** Language the output must be written in (from the vision step or detected). */
   language?: string;
-  /** Set when adding more activities to an existing deck. */
-  existing?: { title: string; types: string[]; count: number };
+  /** Set when adding more activities to an existing deck (or to the part of a
+   *  big deck the earlier model calls already built). */
+  existing?: { title: string; types: string[]; count: number; topics?: string[] };
   /** Extra instructions from the student (topic hint). */
   focus?: string;
 };
@@ -159,7 +160,11 @@ export function buildGenerationUserPrompt({
       : "LANGUAGE: detect the language of the notes below and write everything in that same language.",
     focus ? `The student especially wants to practise: ${focus}` : "",
     existing
-      ? `This is an addition to the existing deck "${existing.title}" which already has ${existing.count} activities (types used so far: ${existing.types.join(", ") || "none"}). Generate ${count} NEW activities on material that was not already covered, and prefer types that are underrepresented.`
+      ? `This is an addition to the existing deck "${existing.title}" which already has ${existing.count} activities (types used so far: ${existing.types.join(", ") || "none"}).${
+          existing.topics?.length
+            ? ` Topics already covered: ${existing.topics.slice(0, 12).join("; ")} — leave them alone.`
+            : ""
+        } Generate ${count} NEW activities on material that was not already covered, and prefer types that are underrepresented.`
       : "",
     "",
     "=== STUDENT NOTES (the only allowed source of facts) ===",