feat: hide-downloaded filter + reset list + config export/import

Three companion features around the 4.5.22 already-downloaded badge.

1. "Hide downloaded" toggle in the VOD filter row. Persisted to
   localStorage so power users who keep it on across sessions don't
   re-flip it on every launch. Filter applies before the title-search
   filter so the match counter stays consistent.

2. "Reset downloaded list" button in a new Backup & Maintenance
   settings card. Confirm-dialog before clearing, IPC returns the
   removed count for a "cleared N entries" toast. Renderer refreshes
   its config copy + re-renders the VOD grid so badges disappear
   immediately. No files are touched.

3. Config export / import via dialog.show*Dialog. Export strips
   client_secret (should never travel as plain text via cloud sync),
   tags the file with __exportVersion + __exportedAt. Import runs
   the JSON through normalizeConfigTemplates so out-of-range fields
   fall back to defaults; if the imported file lacks client_secret,
   the existing value is preserved. After import the renderer reloads
   config + relocalizes if language changed + re-renders streamers /
   settings form / VOD grid.

DE + EN locale strings for every label, button, toast, and confirm
dialog. New backupCardTitle / backupCardIntro section header in
Settings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -257,6 +257,10 @@
                             <option value="duration_asc">Shortest first</option>
                         </select>
                         <span id="vodFilterCount" style="color: var(--text-secondary); font-size:12px; min-width:80px;"></span>
+                        <label id="vodHideDownloadedLabel" style="display:flex; align-items:center; gap:6px; color: var(--text-secondary); font-size:12px; cursor:pointer; user-select:none;" title="">
+                            <input type="checkbox" id="vodHideDownloadedToggle" onchange="onVodHideDownloadedChange()" style="accent-color: var(--accent); cursor:pointer;">
+                            <span id="vodHideDownloadedText">Hide downloaded</span>
+                        </label>
                     </div>
                     <div id="vodBulkBar" class="vod-bulk-bar" style="display:none; align-items:center; gap:10px; padding:8px 12px; background: rgba(145, 70, 255, 0.12); border:1px solid rgba(145, 70, 255, 0.4); border-radius:6px; margin-bottom:12px;">
                         <span id="vodBulkCount" style="color: var(--text); font-size:13px; font-weight:600;">0 selected</span>
@@ -558,6 +562,16 @@
                         <pre id="debugLogOutput" class="log-panel">Lade...</pre>
                     </div>
 
+                    <div class="settings-card">
+                        <h3 id="backupCardTitle">Sicherung &amp; Wartung</h3>
+                        <p id="backupCardIntro" style="color: var(--text-secondary); font-size:13px; margin-bottom:12px; line-height:1.5;">Konfiguration sichern, auf einem anderen Geraet wiederherstellen, oder die Liste der bereits heruntergeladenen VODs zuruecksetzen.</p>
+                        <div class="form-row" style="margin-bottom: 10px; flex-wrap: wrap;">
+                            <button class="btn-secondary" id="btnExportConfig" onclick="exportConfigToFile()">Konfiguration exportieren</button>
+                            <button class="btn-secondary" id="btnImportConfig" onclick="importConfigFromFile()">Konfiguration importieren</button>
+                            <button class="btn-secondary" id="btnResetDownloadedIds" onclick="resetDownloadedIds()">Downloaded-VODs zuruecksetzen</button>
+                        </div>
+                    </div>
+
                     <div class="settings-card">
                         <h3 id="runtimeMetricsTitle">Runtime Metrics</h3>
                         <div class="form-row" style="margin-bottom: 10px; align-items: center;">