Update landing page: switch to Bun, add view transitions and UI polish

- CI: replace Node/npm with Bun
- Add view-transition animations for theme toggle
- Set html background for flash-free theme switches
- UI refinements to download button, brew command, and layout
- Replace package-lock.json with bun.lock

.github/workflows/ci.yml

@@ -16,16 +16,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup Node
-        uses: actions/setup-node@v4
-        with:
-          node-version: 22
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
 
       - name: Install dependencies
-        run: npm ci
+        run: bun install --frozen-lockfile
 
       - name: Typecheck
-        run: npx tsc --noEmit
+        run: bun tsc --noEmit
 
   ui-tests:
     runs-on: self-hosted