feat(stats): show the best move word per game variant

Replace the single "best move" number on the statistics screen with a
full-width per-variant breakdown: the highest-scoring play in each variant
the player has played, drawn as game tiles (a wildcard shows its letter but
no value), with the words and scores right-aligned to shared edges.

- backend: new account_best_move table (PK account_id+variant) keeping the
  main word as JSON tiles {letter,value,blank}; captured at game finish in
  buildStats (blank flags taken from every placed blank — equivalent to the
  final board), upserted in the finish transaction and replaced only by a
  strictly higher-scoring play. Guest/honest-AI games still record nothing.
  GetStats + statsDTO expose best_moves.
- wire: StatsView gains best_moves:[BestMoveView{variant,score,word:[BestMoveTile]}]
  (trailing, backward-compatible); gateway encodeStats + UI codec updated.
- ui: new WordTiles component (board's tile look, fixed px size); Stats.svelte
  drops the maxWord card and adds the full-width best-move card (catalogue
  order, empty variants omitted).
- docs: ARCHITECTURE §9 + schema, FUNCTIONAL (+ru), UI_DESIGN.

Tests: mainWordTiles unit + buildStats end-to-end (inttest) + gateway and UI
codec round-trips (incl. a blank tile) + e2e.

docs/UI_DESIGN.md

@@ -280,8 +280,13 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
   everywhere (score card, lobby row, turn line), the add-friend 🤝 is never drawn, and the chat's
   send field and 🛎️ nudge stay **disabled** while the 🔎 dictionary word-check keeps working.
 - **Statistics** (`screens/Stats.svelte`, the lobby 📊 tab): a 2-column grid of stat
-  cards (wins / losses / draws / games / win-rate / best game / best move) — pure
-  numbers, no charts.
+  cards (wins / losses / draws / games / win-rate / best game) — pure numbers, no
+  charts — followed by a **full-width "best move" card** that breaks the best move
+  down per variant: one row per played variant (catalogue order, empty variants
+  omitted) with the variant name on the left, the **word drawn as game tiles**
+  (`components/WordTiles.svelte`, the board's placed-tile look at a small fixed size;
+  a wildcard shows its letter but no value) right-aligned to a shared edge, and the
+  score right-aligned in its own column.
 - **Profile editing** (`screens/Profile.svelte`): an inline form — display name, a
   **UTC-offset** timezone dropdown (defaulting to the browser's offset), the away
   window as hour + 10-minute dropdowns (24-hour, ≤ 12 h), and block toggles — plus an