feat(view): one top row for the status and backup, transient results as toasts

The status line and the backup banner share one row (the banner appears only when a backup exists). Results and notices
show as toasts that close after 4 s; problems stay until closed. The status line keeps only the empty-state hint.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

index.html

@@ -29,12 +29,15 @@
         </div>
       </header>
 
-      <p id="status" class="status-line" role="status" aria-live="polite">엑셀을 불러오면 교각 형상을 생성합니다.</p>
-      <div id="backup-banner" class="status-line" hidden>
-        <span id="backup-text"></span>
-        <button id="restore-backup" class="btn small" type="button">복구</button>
-        <button id="discard-backup" class="btn small" type="button">버리기</button>
+      <div class="status-row">
+        <p id="status" class="status-line"></p>
+        <div id="backup-banner" class="backup-inline" hidden>
+          <span id="backup-text"></span>
+          <button id="restore-backup" class="btn small" type="button">복구</button>
+          <button id="discard-backup" class="btn small" type="button">버리기</button>
+        </div>
       </div>
+      <div id="toast-host" class="toast-host" role="status" aria-live="polite"></div>
 
       <main class="grid" id="view-shape">
         <aside class="panel piers" aria-labelledby="pier-title">