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/api/client.ts

@@ -14,6 +14,7 @@ export interface Document {
   title: string
   content: string // Tiptap JSON (stringified)
   content_text: string // flattened plain text
+  tone: string // target writing tone; steers LLM advice
   word_count: number
   created_at: string
   updated_at: string
@@ -25,9 +26,25 @@ export interface DocUpdate {
   title?: string
   content?: string
   content_text?: string
+  tone?: string
   word_count?: number
 }
 
+// One sense of a word from the offline dictionary.
+export interface WordMeaning {
+  part_of_speech: string
+  definition: string
+  example?: string
+}
+
+// The offline lookup for one word: a few definition senses and a list of
+// synonyms. Either list may be empty when the word isn't a headword.
+export interface WordInfo {
+  word: string
+  definitions: WordMeaning[]
+  synonyms: string[]
+}
+
 export type SuggestionType = 'grammar' | 'phrasing' | 'idiom' | 'clarity' | 'voice'
 
 // A single LLM-proposed edit. `original` is the source of truth for placement —
@@ -83,6 +100,9 @@ export const api = {
   dismissSuggestion: (id: string) =>
     req<void>(`/suggestions/${id}/dismiss`, { method: 'POST' }),
 
+  // Offline word lookup (definition + synonyms) for the right-click popover.
+  lookupWord: (word: string) => req<WordInfo>(`/word/${encodeURIComponent(word)}`),
+
   // Current deployed build id — changes whenever a new frontend ships. The
   // app polls this to offer a refresh. Bypasses any cache so the answer is live.
   version: () => req<{ version: string }>('/version', { cache: 'no-store' }),