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/db/db.go

@@ -325,6 +325,17 @@ CREATE TABLE vocab_words (
 );
 
 CREATE INDEX idx_vocab_due ON vocab_words(user_id, due_at);
+`,
+		},
+		{
+			// Vocabulary garden, follow-up. Some looked-up words have an English
+			// definition but no Chinese gloss; those produced an unanswerable review
+			// card (review reveals only the gloss). `definition` stores a short
+			// English sense captured at lookup as a fallback "meaning" so such words
+			// are still reviewable.
+			name: "0007_vocab_definition",
+			stmt: `
+ALTER TABLE vocab_words ADD COLUMN definition TEXT NOT NULL DEFAULT '';
 `,
 		},
 	}