feat: streamlink quality preference + per-item notifications + path validation

Three Phase-11 wins.

1. Streamlink stream quality is now configurable. config.streamlink_quality
   defaults to "best" (preserves prior behaviour) but can be set to source,
   1080p60, 720p60, 720p, 480p, or audio_only via a new dropdown in
   Settings -> Download. The chosen quality is passed as STREAMS to
   streamlink with ",best" appended as a fallback so an old VOD lacking
   the chosen rendition still completes. Used by both the queue
   downloadVODPart and the standalone download-clip IPC. The whitelist is
   enforced via normalizeStreamlinkQuality so an arbitrary string in the
   config file falls back to "best".

2. Per-item completion notifications. Default off because long queues
   would spam the OS notifications panel. When enabled (Settings ->
   Queue zwischen App-Starts checkbox area), every successful download
   pops a "{title}" notification whose click brings the window forward
   AND opens shell.showItemInFolder on the produced file (or the
   download folder if the file is gone). The end-of-queue summary
   notification still fires regardless.

3. Download-path writability check on selectFolder. The renderer now
   asks the new check-folder-writable IPC after the user picks a
   folder; if isDownloadPathWritable returns false, a warning toast
   surfaces immediately instead of the next download failing with a
   cryptic "datei zu klein" / "ENOENT" error. Save proceeds anyway —
   the user might be picking a USB-stick path that is offline at the
   moment.

Plus DE + EN locale strings for every label/option/hint, all wired
through applyLanguageToStaticUI for live language switch.

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

src/index.html

@@ -480,6 +480,18 @@
                                 <option value="2" id="parallelDownloads2">2 (Parallel)</option>
                             </select>
                         </div>
+                        <div class="form-group">
+                            <label id="streamlinkQualityLabel">Stream-Qualitaet</label>
+                            <select id="streamlinkQuality">
+                                <option value="best" id="streamlinkQualityBest">Best (Standard)</option>
+                                <option value="source" id="streamlinkQualitySource">Source (Original)</option>
+                                <option value="1080p60" id="streamlinkQuality1080p60">1080p60</option>
+                                <option value="720p60" id="streamlinkQuality720p60">720p60</option>
+                                <option value="720p" id="streamlinkQuality720p">720p</option>
+                                <option value="480p" id="streamlinkQuality480p">480p</option>
+                                <option value="audio_only" id="streamlinkQualityAudio">Audio only</option>
+                            </select>
+                        </div>
                         <div class="form-group">
                             <label id="performanceModeLabel">Performance-Profil</label>
                             <select id="performanceMode">
@@ -505,6 +517,10 @@
                                 <input type="checkbox" id="autoResumeQueueToggle">
                                 <span id="autoResumeQueueLabel">Queue beim Start automatisch fortsetzen</span>
                             </label>
+                            <label style="display:flex; align-items:center; gap:8px; margin-top: 8px;">
+                                <input type="checkbox" id="notifyEachCompletionToggle">
+                                <span id="notifyEachCompletionLabel">Benachrichtigung bei jedem fertigen Download</span>
+                            </label>
                         </div>
                         <div class="form-group">
                             <label id="metadataCacheMinutesLabel">Metadata-Cache (Minuten)</label>