Library-Thumbnails: Auto-Capture + Base64-Preview in der UI

Beim Hinzufuegen oder Importieren eines Library-Items wird automatisch
ein PNG-Thumbnail vom Item generiert (Top-View, 128x128) und in
library/previews/<id>.png abgelegt. Frontend rendert die Previews als
Base64-Data-URIs (sicher gegen WebKit-file://-Restriktionen).

library.py:
- _previews_dir(): legt previews/-Folder an
- _preview_rel_for(asset_rel): predictable PNG-Pfad pro Item
- _capture_thumbnail_of_objects(): hided andere Objekte temporaer,
  switcht auf Top-Parallel, ZoomBoundingBox, CaptureToBitmap → PNG,
  restored Viewport + Hidden-State
- read_preview_data_uri(): liest PNG + encoded als data:image/png;base64
- Hook in convert_to_3dm_via_import (vor Cleanup) + save_selection_to_asset

rhinopanel.py:
- _enrich_library_items_with_previews(): haengt previewDataUri an
  jedes Item das ein preview-Feld hat
- Initial-Params + _send_library + ElementeBridge._cmd_list_library
  liefern angereicherte Items
- _add_library_file + _save_selection_as_library setzen preview-Pfad
  im Item wenn Thumbnail-Datei existiert

Frontend:
- SymbolPicker.ItemPreview: rendert <div backgroundImage> mit Base64-URI
  wenn vorhanden, sonst Icon-Fallback
- ProjectSettingsDialog Symbole-Tab: List-Row + Detail-Identity zeigen
  Thumbnail (32px in Liste, 56px im Detail), Icon nur als Fallback

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/ProjectSettingsDialog.jsx

@@ -1115,8 +1115,8 @@ export default function ProjectSettingsDialog({
                         onClick={() => setSelLib(it.id)}
                         style={{
                           display: 'grid',
-                          gridTemplateColumns: '20px 1fr',
-                          alignItems: 'center', gap: 6,
+                          gridTemplateColumns: '32px 1fr',
+                          alignItems: 'center', gap: 8,
                           padding: '5px 10px',
                           cursor: 'pointer',
                           background: isSel ? 'var(--accent-dim)' : 'transparent',
@@ -1129,9 +1129,21 @@ export default function ProjectSettingsDialog({
                         onMouseLeave={(e) => {
                           if (!isSel) e.currentTarget.style.background = 'transparent'
                         }}>
-                        <Icon name={it.type === 'symbol' ? 'navigation' : 'forest'}
-                              size={13}
-                              style={{ color: 'var(--text-muted)' }} />
+                        <div style={{
+                          width: 32, height: 32, borderRadius: 4,
+                          background: it.previewDataUri
+                            ? `url("${it.previewDataUri}") center/contain no-repeat, var(--bg-input)`
+                            : 'var(--bg-input)',
+                          border: '1px solid var(--border-light)',
+                          display: 'flex', alignItems: 'center', justifyContent: 'center',
+                          flexShrink: 0,
+                        }}>
+                          {!it.previewDataUri && (
+                            <Icon name={it.type === 'symbol' ? 'navigation' : 'forest'}
+                                  size={14}
+                                  style={{ color: 'var(--text-muted)' }} />
+                          )}
+                        </div>
                         <div style={{ minWidth: 0 }}>
                           <div style={{ fontSize: 11,
                                          color: 'var(--text-primary)',
@@ -1177,10 +1189,22 @@ export default function ProjectSettingsDialog({
                   return (
                     <>
                       <DetailSection title="Identität">
-                        <div style={{ display: 'flex', alignItems: 'center', gap: 8 }}>
-                          <Icon name={it.type === 'symbol' ? 'navigation' : 'forest'}
-                                size={28}
-                                style={{ color: 'var(--accent)' }} />
+                        <div style={{ display: 'flex', alignItems: 'center', gap: 10 }}>
+                          <div style={{
+                            width: 56, height: 56, borderRadius: 6,
+                            background: it.previewDataUri
+                              ? `url("${it.previewDataUri}") center/contain no-repeat, var(--bg-input)`
+                              : 'var(--bg-input)',
+                            border: '1px solid var(--border)',
+                            display: 'flex', alignItems: 'center', justifyContent: 'center',
+                            flexShrink: 0,
+                          }}>
+                            {!it.previewDataUri && (
+                              <Icon name={it.type === 'symbol' ? 'navigation' : 'forest'}
+                                    size={22}
+                                    style={{ color: 'var(--accent)' }} />
+                            )}
+                          </div>
                           <input type="text" value={it.name || ''}
                             onChange={(ev) => {
                               const nm = ev.target.value