Editor: document tone, right-click word lookup, expanded stats

Four enhancements to make the editor fit real school usage:

- Per-document tone (academic/professional/casual/humorous/creative/
  persuasive/general): new documents.tone column (migration 0002), threaded
  through the docs API, a bilingual ToneSelect dropdown on the title row, and
  injected into the grammar-checkpoint LLM prompt so advice fits the register.
  The voice pass stays tone-agnostic.

- Right-click word lookup: a new offline `lexicon` package serves definitions
  (Wordset, modern ESL-friendly glosses) and synonyms (WordNet synsets first,
  then frequency+stopword-ranked Moby for breadth) from gzipped embedded data,
  behind /api/word/{word} with light morphology. The WordCard popover shows the
  definition and tappable synonym pills that swap the word in place.

- Expanded writing stats: clicking the word count opens a StatsPanel with page
  count, sentences, paragraphs, reading time, average word length, word variety,
  and Flesch-Kincaid reading level — all computed client-side.

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

internal/lexicon/lexicon_test.go

@@ -0,0 +1,76 @@
+package lexicon
+
+import "testing"
+
+func TestLookupKnownWord(t *testing.T) {
+	l := New()
+	res, err := l.Lookup("happy")
+	if err != nil {
+		t.Fatalf("Lookup: %v", err)
+	}
+	if len(res.Definitions) == 0 {
+		t.Errorf("expected definitions for %q, got none", "happy")
+	}
+	if len(res.Synonyms) == 0 {
+		t.Errorf("expected synonyms for %q, got none", "happy")
+	}
+	if len(res.Synonyms) > maxSynonyms {
+		t.Errorf("synonyms not capped: got %d, want <= %d", len(res.Synonyms), maxSynonyms)
+	}
+	if len(res.Definitions) > maxDefinitions {
+		t.Errorf("definitions not capped: got %d, want <= %d", len(res.Definitions), maxDefinitions)
+	}
+}
+
+func TestLookupMorphology(t *testing.T) {
+	l := New()
+	// Inflected forms should resolve to their base entry via candidates().
+	for _, w := range []string{"running", "studies", "boxes", "quickly", "stopped"} {
+		res, err := l.Lookup(w)
+		if err != nil {
+			t.Fatalf("Lookup(%q): %v", w, err)
+		}
+		if len(res.Definitions) == 0 && len(res.Synonyms) == 0 {
+			t.Errorf("expected some result for inflected %q, got nothing", w)
+		}
+	}
+}
+
+func TestLookupUnknownWord(t *testing.T) {
+	l := New()
+	res, err := l.Lookup("zzzxqqq")
+	if err != nil {
+		t.Fatalf("Lookup: %v", err)
+	}
+	if len(res.Definitions) != 0 || len(res.Synonyms) != 0 {
+		t.Errorf("expected empty result for nonsense word, got %+v", res)
+	}
+	// Empty result must still serialize as [] not null for the frontend.
+	if res.Definitions == nil || res.Synonyms == nil {
+		t.Errorf("empty slices must be non-nil for JSON []: %+v", res)
+	}
+}
+
+func TestCandidates(t *testing.T) {
+	cases := map[string]string{
+		"cats":    "cat",
+		"studies": "study",
+		"running": "run",
+		"hoped":   "hope",
+		"quickly": "quick",
+		"happily": "happy",
+	}
+	for word, want := range cases {
+		got := candidates(word)
+		found := false
+		for _, c := range got {
+			if c == want {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("candidates(%q) = %v, missing expected base %q", word, got, want)
+		}
+	}
+}