Add viewer database import and clarify backup actions.

Pairs export with validated .db restore, documents the difference from game
save imports, and renames the JSON upload button to Import game backup.

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/index.html

@@ -40,10 +40,21 @@
           </select>
         </label>
         <label class="upload-btn">
-          <span data-i18n="actions.importBackup">Import backup</span>
+          <span data-i18n="actions.importBackup">Import game backup</span>
           <input type="file" id="file-upload" accept=".json" hidden>
         </label>
-        <a class="upload-btn export-btn" id="export-viewer" href="#" data-i18n="actions.exportViewer">Export viewer</a>
+        <div class="sidebar-backup-block">
+          <p class="sidebar-backup-label" data-i18n="viewerDb.title">Viewer backup</p>
+          <a class="upload-btn export-btn" id="export-viewer" href="#" data-i18n="actions.exportViewer">Export viewer</a>
+          <label class="upload-btn">
+            <span data-i18n="actions.importViewer">Import viewer</span>
+            <input type="file" id="viewer-db-upload" accept=".db,application/octet-stream" hidden>
+          </label>
+          <details class="sidebar-backup-help">
+            <summary data-i18n="viewerDb.helpTitle">What is this?</summary>
+            <p data-i18n="viewerDb.helpBody">The viewer stores imported game backups, history charts, and goals in a SQLite database on the server. Export downloads that database as a backup. Import restores it and replaces all current viewer data. This is not the same as importing a game save (.json).</p>
+          </details>
+        </div>
       </div>
     </aside>