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

web/src/index.css

@@ -156,6 +156,13 @@ button, a, input {
   animation: petal-suggestion-in 200ms ease both;
 }
 
+/* --- Word lookup popover ----------------------------------------------------
+   Right-click a word for its dictionary definition and synonyms. Same soft card
+   treatment as the spelling popover; synonym pills swap the word on click. */
+.petal-word-card {
+  animation: petal-suggestion-in 200ms ease both;
+}
+
 /* --- Accept confetti --------------------------------------------------------
    A tiny CSS-only burst played where a suggestion is accepted: four colored
    dots spray up-and-out, then fade. No JS animation — each dot reads its