feat(clip): add Parts-format preset to Trim-Clip dialog

The Trim-Clip filename-format radio group only offered three presets
(simple, timestamp, custom template). Users who organise their archive
with the global filename_template_parts pattern (e.g.
08.05.2026_Part07.mp4) had to switch to "custom template" and retype
{date}_Part{part_padded}.mp4 every time.

New "parts" preset:
- index.html: 4th radio option, span#formatParts for the live preview
- types.ts + renderer-globals.d.ts: filenameFormat union extended
- main.ts: makeClipFilename branch produces ${dateStr}_Part${padded}.mp4;
  sanitizeCustomClip whitelists "parts" so persisted queue items with
  the new format survive a restart
- renderer.ts: getSelectedFilenameFormat returns "parts"; live preview
  via partNum.padStart(2, "0")
- DE/EN locales: clips.formatParts label

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

src/index.html

@@ -106,6 +106,11 @@
                         style="width: 18px; height: 18px; accent-color: #9146FF;">
                     <span id="formatTimestamp" style="color: #aaa;">01.02.2026_CLIP_00-00-00_1.mp4 (mit Zeitstempel)</span>
                 </label>
+                <label style="display: flex; align-items: center; gap: 10px; cursor: pointer; margin-bottom: 8px;">
+                    <input type="radio" name="filenameFormat" value="parts" onchange="updateFilenameExamples()"
+                        style="width: 18px; height: 18px; accent-color: #9146FF;">
+                    <span id="formatParts" style="color: #aaa;">01.02.2026_Part01.mp4 (Parts-Format)</span>
+                </label>
                 <label style="display: flex; align-items: center; gap: 10px; cursor: pointer; margin-bottom: 10px;">
                     <input type="radio" name="filenameFormat" value="template" onchange="updateFilenameExamples()"
                         style="width: 18px; height: 18px; accent-color: #9146FF;">