fix(security): address critical and high findings from security audit

Fix stored XSS in tasks (titles/subtasks) and settings (member list)
by applying escHtml(). Harden trust proxy to loopback default, add
OAuth state parameter for Google Calendar CSRF protection, sanitize
CSV export against formula injection, invalidate sessions on user
deletion, restrict usernames to alphanumeric chars, and require admin
role for calendar sync triggers.

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

server/routes/budget.js

@@ -147,14 +147,19 @@ router.get('/export', (req, res) => {
     `).all(from, to);
 
     const header = 'Datum,Titel,Betrag,Kategorie,Wiederkehrend,Erstellt von\n';
+    const csvSafe = (val) => {
+      let s = String(val || '').replace(/"/g, '""');
+      if (/^[=+\-@\t\r]/.test(s)) s = "'" + s;
+      return `"${s}"`;
+    };
     const rows   = entries.map((e) =>
       [
         e.date,
-        `"${(e.title || '').replace(/"/g, '""')}"`,
+        csvSafe(e.title),
         e.amount.toFixed(2).replace('.', ','),
         e.category,
         e.is_recurring ? 'Ja' : 'Nein',
-        `"${(e.creator_name || '').replace(/"/g, '""')}"`,
+        csvSafe(e.creator_name),
       ].join(',')
     ).join('\n');