Code-review fixes for collocation coach + vocab garden

Correctness:
- useCheckpoint: clear the busy flag unconditionally so overlapping
  explicit passes don't strand each other's spinner; explicit actions
  now also supersede a queued auto-check and clear the stranded
  "checking" dot. Deduped runVoice/runCollocation into runExplicitPass.
- EditorCore: token-guard the auto-capture so a late capture can't
  resurrect a removed word; move toggleSaveWord side effects out of the
  setWordInfo updater (StrictMode double-fire); fix sentenceAround offset
  desync via shared exampleAt (textBetween + parentOffset, single resolve);
  optimistic saved state so the heart doesn't flash unsaved.
- vocab capture: normalize word to lower+trim (matches lexicon) so
  "Apple"/"apple" don't make duplicate cards; check rows.Err() in queryList.
- GardenPanel: Promise.allSettled so a /due failure doesn't blank the
  whole garden; scrim click during review ends the review (mirrors Esc);
  gate footer on !error; O(1) due lookup via a Set.

Features requested in review:
- Definition-only review card: add vocab_words.definition (migration
  0007) as an English fallback meaning, threaded through capture and used
  by review/garden when there's no Chinese gloss.
- Scheduler caps: maxEase 3.0 + maxInterval 365d so "easy" growth can't
  push a word out of rotation for years.

Tests: TestCaptureCaseInsensitive, TestCaptureStoresDefinitionFallback,
TestCapsBoundGrowth. go build/vet/test, tsc, vitest 51/51, vite build clean.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

internal/vocab/handlers_test.go

@@ -141,6 +141,48 @@ func TestCaptureValidation(t *testing.T) {
 	}
 }
 
+// TestCaptureStoresDefinitionFallback proves a word with an English definition
+// but no Chinese gloss still round-trips its definition, so review has a meaning
+// to reveal instead of a blank card.
+func TestCaptureStoresDefinitionFallback(t *testing.T) {
+	srv, _ := newTestServer(t)
+	rec := do(t, srv, http.MethodPost, "/vocab",
+		`{"word":"ineffable","definition":"too great to be expressed in words"}`)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("capture: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var w Word
+	_ = json.Unmarshal(rec.Body.Bytes(), &w)
+	if w.Gloss != "" {
+		t.Fatalf("expected no gloss, got %q", w.Gloss)
+	}
+	if w.Definition != "too great to be expressed in words" {
+		t.Fatalf("definition not stored, got %q", w.Definition)
+	}
+}
+
+// TestCaptureCaseInsensitive proves "Apple" (sentence start) and "apple"
+// (mid-line) land in the SAME garden card — capture normalizes to lower+trim
+// like the lexicon, so the idempotent upsert isn't defeated by casing.
+func TestCaptureCaseInsensitive(t *testing.T) {
+	srv, _ := newTestServer(t)
+	if rec := do(t, srv, http.MethodPost, "/vocab", `{"word":"Apple","gloss":"苹果"}`); rec.Code != http.StatusCreated {
+		t.Fatalf("capture Apple: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if rec := do(t, srv, http.MethodPost, "/vocab", `{"word":"  apple ","gloss":"苹果"}`); rec.Code != http.StatusCreated {
+		t.Fatalf("capture apple: code=%d body=%s", rec.Code, rec.Body)
+	}
+	rec := do(t, srv, http.MethodGet, "/vocab", "")
+	var all []Word
+	_ = json.Unmarshal(rec.Body.Bytes(), &all)
+	if len(all) != 1 {
+		t.Fatalf("Apple/apple should be one card, got %d", len(all))
+	}
+	if all[0].Word != "apple" {
+		t.Fatalf("word should be normalized to lowercase, got %q", all[0].Word)
+	}
+}
+
 // TestDocLinkSurvivesDocDelete proves the ON DELETE SET NULL keeps a word in the
 // garden when its source document is removed.
 func TestDocLinkSurvivesDocDelete(t *testing.T) {