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.

ui/src/components/WordTiles.svelte

@@ -0,0 +1,53 @@
+<script lang="ts">
+  // A best-move word drawn as a row of game tiles, mirroring the board's placed-tile
+  // look (letter top-left, point value bottom-right) at a small fixed size. A blank tile
+  // shows its letter but no value, exactly as on the board. Letters are upper-cased for
+  // display. The tile values ride on each tile, so this renders without the variant's
+  // alphabet table (which the statistics screen has not cached).
+  import type { BestMoveTile } from '../lib/model';
+
+  let { word }: { word: BestMoveTile[] } = $props();
+
+  const label = $derived(word.map((t) => t.letter).join('').toUpperCase());
+</script>
+
+<span class="word" aria-label={label}>
+  {#each word as tile, i (i)}
+    <span class="tile" class:blank={tile.blank} aria-hidden="true">
+      <span class="letter">{tile.letter.toUpperCase()}</span>
+      {#if !tile.blank}<span class="val">{tile.value}</span>{/if}
+    </span>
+  {/each}
+</span>
+
+<style>
+  .word {
+    display: inline-flex;
+    gap: 2px;
+  }
+  .tile {
+    position: relative;
+    flex: none;
+    width: 22px;
+    height: 22px;
+    background: var(--tile-bg);
+    color: var(--tile-text);
+    border-radius: 3px;
+    box-shadow: inset 0 -2px 0 var(--tile-edge);
+  }
+  .letter {
+    position: absolute;
+    top: 6%;
+    left: 11%;
+    font-size: 12px;
+    font-weight: 700;
+    line-height: 1;
+  }
+  .val {
+    position: absolute;
+    right: 8%;
+    bottom: 3%;
+    font-size: 7px;
+    font-weight: 600;
+  }
+</style>