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

.github/dependabot.yml

@@ -1,32 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "npm"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-      time: "06:00"
-    open-pull-requests-limit: 5
-    reviewers:
-      - "tuan-dat-tran"
-    labels:
-      - "dependencies"
-    commit-message:
-      prefix: "deps"
-      include: "scope"
-      
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-      time: "06:00"
-    open-pull-requests-limit: 3
-    reviewers:
-      - "tuan-dat-tran"
-    labels:
-      - "dependencies"
-      - "github-actions"
-    commit-message:
-      prefix: "ci"
-      include: "scope"