Grade against the question: stop requiring details the question never asked for

A short-answer case: "Why was Kryštof Harant executed?" with the student
answering "because he took part in the anti-Habsburg uprising" was failed
for not naming the year 1621 — a detail only present in the reference
answer, never asked by the question. The generator had put the date into
keyPoints, and the grader treated every key point as mandatory.

- Grading prompt: the QUESTION defines what is required. A key point
  counts only when the question asks for it or the answer is wrong or
  incomplete without it; extra detail in the reference answer (dates,
  places, names, numbers) is a bonus — never listed as missing, never a
  reason to fail. An accepted answer must return an empty "missing"
  array.
- Generation spec: short_answer keyPoints must contain only the ideas
  the question asks for (a "why" question requires the reason, not the
  year).
- gradeShortAnswer drops any stray "missing" entries when correct is
  true, so the UI can never show a gap on an accepted answer.

Verified: mock-Ollama integration test 9/9 through the real /api/grade
route (new rules sent upstream, incorrect answers keep their missing
point, accepted answers normalise to missing: []), regression 74/74,
mobile 51/51, i18n 27/27. Lint + build clean. SW freezmo-v18.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v17";
+const CACHE_VERSION = "freezmo-v18";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;