ci: add dependabot config and PR redirect workflow

- dependabot.yml: target pre-release branch for npm and cargo updates
- redirect-pr-to-prerelease.yml: auto-redirect PRs to main → pre-release
  with a friendly comment explaining the workflow

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    target-branch: "pre-release"
+
+  - package-ecosystem: "cargo"
+    directory: "/src-tauri"
+    schedule:
+      interval: "weekly"
+    target-branch: "pre-release"