feat: improve dashboard and connection workflows

- add connection delete endpoint and update connection list handling

- expand dashboard visualization behavior

- update helpers/header and project TODO tracking

app/modules/connections/delete.php

@@ -0,0 +1,41 @@
+<?php
+/**
+ * app/modules/connections/delete.php
+ *
+ * Loescht eine Verbindung und leitet zur Liste zurueck.
+ */
+
+$connectionId = (int)($_GET['id'] ?? $_POST['id'] ?? 0);
+
+if ($connectionId <= 0) {
+    $_SESSION['error'] = 'Ungueltige Verbindungs-ID';
+    header('Location: ?module=connections&action=list');
+    exit;
+}
+
+$connection = $sql->single(
+    "SELECT id FROM connections WHERE id = ?",
+    "i",
+    [$connectionId]
+);
+
+if (!$connection) {
+    $_SESSION['error'] = 'Verbindung nicht gefunden';
+    header('Location: ?module=connections&action=list');
+    exit;
+}
+
+$rows = $sql->set(
+    "DELETE FROM connections WHERE id = ?",
+    "i",
+    [$connectionId]
+);
+
+if ($rows > 0) {
+    $_SESSION['success'] = 'Verbindung geloescht';
+} else {
+    $_SESSION['error'] = 'Verbindung konnte nicht geloescht werden';
+}
+
+header('Location: ?module=connections&action=list');
+exit;