feat: Settings in Datenbank speichern statt localStorage

- Neue Spalte 'settings' in users-Tabelle (Migration via TryAddColumn)
- SettingsEndpoint: GET/PUT /api/settings
- DatabaseService: GetUserSettings() + UpdateUserSettings()
- Frontend: currentSettings als globaler State in app.js
- settings-page.js: localStorage komplett entfernt, nutzt API
- Settings werden beim App-Start aus DB geladen und angewendet

Services/DatabaseService.cs

@@ -162,6 +162,7 @@ public class DatabaseService
         TryAddColumn(conn, "agents", "user_id", "TEXT");
         TryAddColumn(conn, "users", "is_admin", "INTEGER NOT NULL DEFAULT 0");
         TryAddColumn(conn, "users", "persona", "TEXT");
+        TryAddColumn(conn, "users", "settings", "TEXT");
 
         conn.Execute(@"
             CREATE TABLE IF NOT EXISTS agent_skills (
@@ -792,6 +793,22 @@ public class DatabaseService
             new { userId, displayName, persona, now = DateTime.UtcNow.ToString("o") });
     }
 
+    public async Task<string> GetUserSettings(string userId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QuerySingleOrDefaultAsync<string>(
+            "SELECT settings FROM users WHERE user_id = @userId",
+            new { userId });
+    }
+
+    public async Task UpdateUserSettings(string userId, string settings)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "UPDATE users SET settings = @settings, updated_at = @now WHERE user_id = @userId",
+            new { userId, settings, now = DateTime.UtcNow.ToString("o") });
+    }
+
     // ── Telegram ──
 
     public async Task<string> CreateTelegramLinkCode(string userId, string sessionId, string agentId)