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.go

@@ -0,0 +1,205 @@
+package lexicon
+
+import (
+	"bytes"
+	"compress/gzip"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+	"sync"
+)
+
+// Meaning is one sense of a word: its part of speech, the gloss, and an optional
+// usage example. The frontend renders a few of these in the word popover.
+type Meaning struct {
+	PartOfSpeech string `json:"part_of_speech"`
+	Definition   string `json:"definition"`
+	Example      string `json:"example,omitempty"`
+}
+
+// Result is the full lookup for one word. Either list may be empty (the word
+// isn't a headword in that dataset); the frontend handles a partial or empty
+// result gracefully.
+type Result struct {
+	Word        string    `json:"word"`
+	Definitions []Meaning `json:"definitions"`
+	Synonyms    []string  `json:"synonyms"`
+}
+
+// maxSynonyms caps how many synonyms we hand the popover, even though the dataset
+// stores up to ~50 per word — a long flat wall of words overwhelms more than it
+// helps, especially for an ESL reader scanning for the right fit.
+const maxSynonyms = 16
+
+// maxDefinitions caps the senses shown so the popover stays a glance, not an
+// essay.
+const maxDefinitions = 4
+
+// Lexicon holds the lazily-loaded datasets. The maps are populated once, on the
+// first Lookup, behind a sync.Once so startup stays instant and a load error is
+// remembered rather than retried on every request.
+type Lexicon struct {
+	once     sync.Once
+	loadErr  error
+	defs     map[string][][]string // word → [[pos, def, example], …]
+	synonyms map[string][]string   // word → [synonym, …]
+}
+
+// New returns a Lexicon. The datasets aren't read until the first Lookup.
+func New() *Lexicon { return &Lexicon{} }
+
+func (l *Lexicon) load() {
+	l.once.Do(func() {
+		if err := gunzipJSON(definitionsGz, &l.defs); err != nil {
+			l.loadErr = fmt.Errorf("load definitions: %w", err)
+			return
+		}
+		if err := gunzipJSON(synonymsGz, &l.synonyms); err != nil {
+			l.loadErr = fmt.Errorf("load synonyms: %w", err)
+			return
+		}
+	})
+}
+
+// Lookup returns the definition senses and synonyms for word. It first tries the
+// word as written (lowercased), then a few simple morphological reductions
+// (plurals, -ed/-ing/-ly) so "running" or "happily" still resolve. A word found
+// in neither dataset yields a Result with empty lists (not an error).
+func (l *Lexicon) Lookup(word string) (Result, error) {
+	l.load()
+	if l.loadErr != nil {
+		return Result{}, l.loadErr
+	}
+
+	norm := strings.ToLower(strings.TrimSpace(word))
+	res := Result{Word: word, Definitions: []Meaning{}, Synonyms: []string{}}
+	if norm == "" {
+		return res, nil
+	}
+
+	if raw := lookupDefs(l.defs, norm); raw != nil {
+		for _, m := range raw {
+			res.Definitions = append(res.Definitions, toMeaning(m))
+			if len(res.Definitions) >= maxDefinitions {
+				break
+			}
+		}
+	}
+
+	if syns := lookupSyns(l.synonyms, norm); syns != nil {
+		if len(syns) > maxSynonyms {
+			syns = syns[:maxSynonyms]
+		}
+		res.Synonyms = append(res.Synonyms, syns...)
+	}
+
+	return res, nil
+}
+
+// lookupDefs / lookupSyns walk the candidate forms of a word and return the
+// first dataset hit. They're separate (rather than a generic helper) only
+// because the two maps have different value types.
+func lookupDefs(m map[string][][]string, word string) [][]string {
+	for _, c := range candidates(word) {
+		if v, ok := m[c]; ok {
+			return v
+		}
+	}
+	return nil
+}
+
+func lookupSyns(m map[string][]string, word string) []string {
+	for _, c := range candidates(word) {
+		if v, ok := m[c]; ok {
+			return v
+		}
+	}
+	return nil
+}
+
+// candidates returns the lemma forms to try, in priority order: the word itself,
+// then conservative de-inflections. This is deliberately lightweight — a full
+// stemmer would over-reduce ("business" → "busy") and surface wrong entries; a
+// handful of common English suffix rules covers the everyday cases without a
+// dependency. Duplicates are fine (map lookup is cheap); order is what matters.
+func candidates(word string) []string {
+	out := []string{word}
+	add := func(s string) {
+		if len(s) >= 2 && s != word {
+			out = append(out, s)
+		}
+	}
+
+	switch {
+	case strings.HasSuffix(word, "ies"): // studies → study
+		add(word[:len(word)-3] + "y")
+	case strings.HasSuffix(word, "es"): // boxes → box, wishes → wish
+		add(word[:len(word)-2])
+		add(word[:len(word)-1])
+	case strings.HasSuffix(word, "s"): // cats → cat
+		add(word[:len(word)-1])
+	}
+
+	if strings.HasSuffix(word, "ing") { // running → run, making → make
+		stem := word[:len(word)-3]
+		add(stem)
+		add(stem + "e")
+		add(undouble(stem))
+	}
+	if strings.HasSuffix(word, "ed") { // hoped → hope, stopped → stop
+		stem := word[:len(word)-2]
+		add(stem)
+		add(word[:len(word)-1])
+		add(undouble(stem))
+	}
+	if strings.HasSuffix(word, "ly") { // happily handled above via ies path? no — quickly → quick
+		add(word[:len(word)-2])
+	}
+	if strings.HasSuffix(word, "ily") { // happily → happy
+		add(word[:len(word)-3] + "y")
+	}
+
+	return out
+}
+
+// undouble collapses a doubled final consonant (stopp → stop, runn → run) so the
+// -ed/-ing stems of doubled-consonant verbs resolve to their base form.
+func undouble(stem string) string {
+	n := len(stem)
+	if n >= 2 && stem[n-1] == stem[n-2] {
+		return stem[:n-1]
+	}
+	return stem
+}
+
+// toMeaning maps a compact [pos, def, example] triple from the dataset onto the
+// JSON-friendly Meaning. The dataset always stores three elements, but we guard
+// the length so a malformed row can't panic.
+func toMeaning(m []string) Meaning {
+	var out Meaning
+	if len(m) > 0 {
+		out.PartOfSpeech = m[0]
+	}
+	if len(m) > 1 {
+		out.Definition = m[1]
+	}
+	if len(m) > 2 {
+		out.Example = m[2]
+	}
+	return out
+}
+
+// gunzipJSON decompresses gz and decodes the JSON into v.
+func gunzipJSON(gz []byte, v any) error {
+	r, err := gzip.NewReader(bytes.NewReader(gz))
+	if err != nil {
+		return err
+	}
+	defer r.Close()
+	data, err := io.ReadAll(r)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(data, v)
+}