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.

backend/internal/postgres/migrations/00009_best_move.sql

@@ -0,0 +1,25 @@
+-- +goose Up
+-- Per-account, per-variant best single move: the highest-scoring play the account has
+-- ever made in each game variant. It exists so the statistics screen can show the word
+-- itself broken down by variant, not just the dimensionless aggregate
+-- account_stats.max_word_points. tiles is the move's main word as an ordered JSON array of
+-- {letter, value, blank} objects (value 0 and blank true for a wildcard), so the client
+-- renders it as game tiles without needing the variant's alphabet table. score is the
+-- play's total points (every word it formed plus the all-tiles bonus), matching
+-- max_word_points. A row is replaced only by a strictly higher-scoring play. It is written
+-- at game finish alongside account_stats; guest and honest-AI games never record statistics,
+-- so they never write here. See docs/ARCHITECTURE.md §9.
+SET search_path = backend, pg_catalog;
+
+CREATE TABLE account_best_move (
+    account_id uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    variant    text        NOT NULL,
+    score      integer     NOT NULL,
+    tiles      jsonb       NOT NULL,
+    updated_at timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (account_id, variant)
+);
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+DROP TABLE IF EXISTS account_best_move;