Session-Cards: buildSessionCard() Helper + Picker-Modal für Cronjob-Formular

- buildSessionCard(session, agent) Helper in ui.js (analog zu buildAgentCard)
  Erzeugt .card mit Avatar, Name, Badges (Agent, Kontext, Telegram) und Meta (Nachrichten, Archiv, Workspace)
- Dashboard refactored: Session-Cards nutzen buildSessionCard() statt inline HTML
- Cronjob-Formular: <select> für Session und Benachrichtigungs-Session durch
  klickbare Session-Cards + Picker-Modal ersetzt
- openSessionPickerModal() mit filterWithAgent-Option (Session-Dropdown nur
  Sessions mit Agent, Notify-Dropdown alle Sessions)
- CSS: .session-select-card und .session-select-change Klassen analog zu agent-*

wwwroot/css/form.css

@@ -297,9 +297,10 @@ label.skill-item {
   margin-left: auto;
 }
 
-/* ── Agent Select Card (in Session-Formular) ── */
+/* ── Select Cards (Agent / Session in Formularen) ── */
 
-.agent-select-card {
+.agent-select-card,
+.session-select-card {
   display: flex;
   align-items: center;
   gap: var(--box-spacing);
@@ -311,30 +312,35 @@ label.skill-item {
   transition: border-color var(--transition-duration);
 }
 
-.agent-select-card:hover {
+.agent-select-card:hover,
+.session-select-card:hover {
   border-color: var(--primary);
 }
 
-.agent-select-card .card-image {
+.agent-select-card .card-image,
+.session-select-card .card-image {
   flex: 0 0 4em;
   height: 4em;
 }
 
-.agent-select-empty {
+.agent-select-empty,
+.session-select-empty {
   opacity: var(--semi-transparent);
   display: flex;
   align-items: center;
   gap: var(--badge-spacing);
 }
 
-.agent-select-change {
+.agent-select-change,
+.session-select-change {
   margin-left: auto;
   opacity: var(--semi-transparent);
   flex-shrink: 0;
   transition: opacity var(--transition-duration);
 }
 
-.agent-select-card:hover .agent-select-change {
+.agent-select-card:hover .agent-select-change,
+.session-select-card:hover .session-select-change {
   opacity: 1;
   color: var(--primary);
 }