fix: correct serve config assignment and clean up README

- serve.json: now uses ${TS_CERT_DOMAIN} with 443 (Tailscale default)
- serve.headscale.json: uses hardcoded example domain (Headscale)
- README: replace all {{service}} placeholders with vikunja
- Update logo to selfh.st icon
- Harden .gitignore

tailscale/config/serve.json

@@ -2,15 +2,28 @@
   "TCP": {
     "80": {
       "HTTP": true
+    },
+    "443": {
+      "HTTPS": true
     }
   },
   "Web": {
-    "vikunja.example.com:80": {
+    "${TS_CERT_DOMAIN}:80": {
+      "Handlers": {
+        "/": {
+          "Proxy": "http://127.0.0.1:3456"
+        }
+      }
+    },
+    "${TS_CERT_DOMAIN}:443": {
       "Handlers": {
         "/": {
           "Proxy": "http://127.0.0.1:3456"
         }
       }
     }
+  },
+  "AllowFunnel": {
+    "${TS_CERT_DOMAIN}:443": false
   }
 }