fix(ci): replace dependabot with renovate for Gitea compatibility

Dependabot only works on GitHub. This repo is hosted on Gitea which
requires Renovate for dependency updates.

- Remove .github/dependabot.yml (unused on Gitea)
- Add renovate.json with recommended config
- Configure npm package updates for / and /backend
- Configure GitHub Actions workflow updates
- Enable automerge for dev dependency minor/patch updates

renovate.json

@@ -0,0 +1,37 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ],
+  "timezone": "Europe/Berlin",
+  "schedule": ["on monday before 10:00am"],
+  "prConcurrentLimit": 5,
+  "prHourlyLimit": 2,
+  "labels": ["dependencies"],
+  "commitMessagePrefix": "deps",
+  "commitMessageAction": "update",
+  "packageRules": [
+    {
+      "matchManagers": ["npm"],
+      "rangeStrategy": "bump",
+      "semanticCommitType": "deps",
+      "semanticCommitScope": "{{packageFileDir}}"
+    },
+    {
+      "matchManagers": ["github-actions"],
+      "semanticCommitType": "ci",
+      "semanticCommitScope": "actions",
+      "groupName": "github-actions"
+    },
+    {
+      "matchDepTypes": ["devDependencies"],
+      "matchUpdateTypes": ["minor", "patch"],
+      "automerge": true
+    }
+  ],
+  "lockFileMaintenance": {
+    "enabled": true,
+    "schedule": ["on the first day of the month"]
+  },
+  "postUpdateOptions": ["npmDedupe"]
+}