feat: auto-merge resumed live-recording parts via ffmpeg concat

Closes the loop on 4.6.13 auto-resume. A streamlink restart between
two parts produces N separate .mp4 files for what is logically a
single recording, which is fine for reliability but inconvenient
for watching back. Opt-in flag flips that into a single stitched
file post-recording.

concatVideoFiles(inputs, output) writes a temp concat list and runs
ffmpeg with the concat demuxer in copy mode — no re-encode, the
parts get container-stitched in seconds even for multi-hour
recordings. The merged output is named "{base}_merged.mp4" so it
sits next to the parts without colliding.

Two independent toggles:
- auto_merge_resumed_parts (off by default) — runs the merge.
- delete_parts_after_merge (off by default) — drops the originals
  ONLY if the merge produced a non-zero output file. Default-off
  means even if ffmpeg silently produced garbage, the parts stay
  around as the source of truth.

If concat fails for any reason (corrupt segment header, codec
mismatch from a stream that changed quality mid-recording, missing
ffmpeg) the failure is non-fatal: we delete the half-written
merged file and keep the parts. The user always has the original
recordings.

Settings card adds the two checkboxes nested under the existing
auto-resume toggle so the relationship is visually obvious.

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

src/index.html

@@ -639,6 +639,14 @@
                                 <input type="checkbox" id="autoResumeLiveRecordingToggle" checked>
                                 <span id="autoResumeLiveRecordingLabel">Live-Aufnahme automatisch fortsetzen wenn Streamlink abbricht (max. 5 Versuche)</span>
                             </label>
+                            <label style="display:flex; align-items:center; gap:8px; margin-top: 8px;">
+                                <input type="checkbox" id="autoMergeResumedPartsToggle">
+                                <span id="autoMergeResumedPartsLabel">Fortgesetzte Aufnahme-Parts automatisch zu einer Datei zusammenfuegen (ffmpeg concat)</span>
+                            </label>
+                            <label style="display:flex; align-items:center; gap:8px; margin-top: 8px; margin-left: 22px;">
+                                <input type="checkbox" id="deletePartsAfterMergeToggle">
+                                <span id="deletePartsAfterMergeLabel">Einzelne Parts nach erfolgreichem Merge loeschen</span>
+                            </label>
                         </div>
                         <div class="form-group">
                             <label id="metadataCacheMinutesLabel">Metadata-Cache (Minuten)</label>