chore: upgrade GitHub Actions to Node.js 24 compatible versions

- actions/setup-node@v3 → @v4
- actions/checkout@v3 → @v5 (pr.yml)
- peaceiris/actions-gh-pages@v3 → @v4
- node-version 18 → 22

Node.js 20 actions are deprecated and will be forced to Node.js 24
starting June 2, 2026.

Closes #30

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MantisClone

.github/workflows/deploy.yml

@@ -10,15 +10,15 @@ jobs:
     permissions:
       contents: write  # Required to push historical snapshot to main
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           ref: main  # Checkout main branch, not the release tag
           fetch-depth: 0  # Fetch all history to access previous versions
 
       - name: Setup Node.js
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v6
         with:
-          node-version: "18"
+          node-version: "22"
           cache: "npm"
 
       - name: Install dependencies
@@ -142,7 +142,7 @@ jobs:
           fi
 
       - name: Deploy to GitHub Pages
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./dist

.github/workflows/pr.yml

@@ -8,12 +8,12 @@ jobs:
   validate:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       - name: Setup Node.js
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v6
         with:
-          node-version: "18"
+          node-version: "22"
           cache: "npm"
 
       - name: Install dependencies