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/llm/voice.go

@@ -16,7 +16,10 @@ const VoiceInterval = 20 * time.Second
 // for a Tier-1 voice pass and parses the JSON result. It reuses the checkpoint's
 // tolerant parser and a larger token budget, since one pass may flag several
 // passages. Each flag carries a null replacement (awareness-only).
-func RunVoice(ctx context.Context, client LLMClient, contentText string) ([]RawSuggestion, error) {
+// The tone argument is accepted for a uniform pass signature but ignored: voice
+// consistency is judged against the document's own established voice, not an
+// externally-chosen register.
+func RunVoice(ctx context.Context, client LLMClient, contentText, _ string) ([]RawSuggestion, error) {
 	raw, err := client.Complete(ctx, CompletionRequest{
 		Messages:          VoiceMessages(contentText),
 		MaxTokens:         2048,