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/e2e/social.spec.ts

@@ -53,11 +53,15 @@ test('invitations: the lobby shows an invitation and accepting clears it', async
   await expect(page.getByText(/From Kaya/)).toBeHidden();
 });
 
-test('stats screen shows the metrics', async ({ page }) => {
+test('stats screen shows the metrics and the per-variant best move', async ({ page }) => {
   await loginLobby(page);
   await page.getByRole('button', { name: /Stats/ }).click();
   await expect(page.getByText('Win rate')).toBeVisible();
   await expect(page.getByText('Best move')).toBeVisible();
+  // The best move breaks down per played variant, each row labelled by the variant and
+  // ending in the play's score (the word itself renders as game tiles).
+  await expect(page.getByText('Scrabble', { exact: true })).toBeVisible();
+  await expect(page.getByText('134')).toBeVisible();
 });
 
 test('settings hub: tabs switch in place and back returns to the lobby', async ({ page }) => {