feat: calendar rename for all sources, group icon in sidebar, configurable share icon

- Rename: Add google and homeassistant branches to the dblclick-rename
  save() handler; backend already accepted name updates for both sources
- Group icon: Pass groupIconForLocalCal(cal.id) into sidebar entries so
  the group's custom icon (home/heart/work/…) shows instead of always
  defaulting to the people icon
- Share icon: New share_calendar_icon field in UserSettings (+ migration)
  replaces the hardcoded SHARE_ICON SVG; users pick from GROUP_ICON_KEYS
  in Settings → Darstellung via a new icon-picker row

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -789,6 +789,10 @@
             <button class="contrast-btn" data-val="80"><span class="hour-preview">━━━━━</span><span class="contrast-lbl" data-i18n="hour_large">Gross</span></button>
           </div>
 
+          <h4 class="panel-title" style="margin-top:24px">Teilen-Symbol</h4>
+          <p class="panel-desc">Symbol neben deinem geteilten Kalender in der Seitenleiste</p>
+          <div class="group-icon-picker" id="cfg-share-icon"></div>
+
           <h4 class="panel-title" style="margin-top:24px" data-i18n="settings_colors">Farben</h4>
           <div class="form-group">
             <label data-i18n="settings_primary_color">Primärfarbe</label>