fix(frontend): restore Ref document and control contracts

Ref's `.bg0/.bg1/.bg2` set only a background image and `html, body` centre the
texture with vertical tiling, so the extra background colours and the default
tiling are removed. Two ranking views declared unscoped `body` rules that leaked
a 500px min-width and a 21px line-height onto every screen; each page now owns
its own contract. Controls that Ref renders with the Bootstrap/Lumen family get
the shared modifier so the bottom edge moves on hover and while pressed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/game-frontend/src/views/BoardView.vue

@@ -158,7 +158,7 @@ onMounted(() => {
                 </div>
                 <div class="article-submit-row">
                     <div></div>
-                    <button id="submitArticle" class="legacy-button" type="button" @click="submitArticle">등록</button>
+                    <button id="submitArticle" class="legacy-button legacy-button--secondary" type="button" @click="submitArticle">등록</button>
                 </div>
             </section>