ci: render catalog previews on PR (#262)

## What

CI workflow that auto-renders preview thumbnails for new/changed registry blocks and components on pull requests.

**New files:**
- `scripts/generate-catalog-previews.ts` — catalog preview renderer supporting all three registry item types
- `.github/workflows/catalog-previews.yml` — GitHub Actions workflow triggered on PRs touching `registry/blocks/` or `registry/components/`

## Why

Phase B of the catalog plan (PR 8). After this lands, future block/component PRs don't need to manually generate preview images — CI handles it automatically.

## How

The preview script discovers items from the registry directory structure:
- **Examples**: renders `index.html` (same as the existing `generate-template-previews.ts`)
- **Blocks**: renders the block's standalone HTML file directly (e.g., `data-chart.html`)
- **Components**: renders the component's `demo.html` (the demo.html convention from PR 7)

The CI workflow:
1. Detects which blocks/components changed in the PR via `git diff`
2. Renders thumbnails for only the changed items (not the full catalog)
3. Uploads preview PNGs as artifacts

Output goes to `docs/images/catalog/<type>/<name>.{png,mp4}` (separate from the existing `docs/images/templates/` directory).

Supports CLI flags: `--only <name>`, `--type <example|block|component>`, `--skip-video`.

## Test plan

- [x] Script compiles and passes typecheck (`lefthook pre-commit` ran lint + typecheck + format)
- [x] Workflow YAML is valid (standard GitHub Actions syntax, follows existing ci.yml patterns)
- [ ] Full end-to-end test requires Chrome + FFmpeg (runs in CI, not testable locally without producer deps)

.github/workflows/catalog-previews.yml

@@ -0,0 +1,76 @@
+name: Catalog Previews
+
+on:
+  pull_request:
+    paths:
+      - "registry/blocks/**"
+      - "registry/components/**"
+      - "scripts/generate-catalog-previews.ts"
+
+concurrency:
+  group: catalog-previews-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  render-previews:
+    name: Render catalog previews
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+
+      - run: bun install --frozen-lockfile
+
+      - run: bun run build
+
+      # Chrome headless shell for rendering
+      - uses: browser-actions/setup-chrome@v1
+        with:
+          chrome-version: stable
+
+      # FFmpeg for video encoding
+      - uses: FedericoCarboni/setup-ffmpeg@v3
+
+      - name: Render changed block/component previews
+        continue-on-error: true
+        run: |
+          # Find which blocks/components changed in this PR
+          CHANGED_ITEMS=$(git diff --name-only origin/main...HEAD -- registry/blocks/ registry/components/ \
+            | grep -oP '(?<=registry/(blocks|components)/)[^/]+' \
+            | sort -u)
+
+          if [ -z "$CHANGED_ITEMS" ]; then
+            echo "No block/component changes detected."
+            exit 0
+          fi
+
+          echo "Changed items: $CHANGED_ITEMS"
+          FAILED=0
+
+          for item in $CHANGED_ITEMS; do
+            echo "Rendering preview for: $item"
+            if ! npx tsx scripts/generate-catalog-previews.ts --only "$item" --skip-video; then
+              echo "::warning::Failed to render preview for $item"
+              FAILED=$((FAILED + 1))
+            fi
+          done
+
+          if [ "$FAILED" -gt 0 ]; then
+            echo "::warning::$FAILED item(s) failed to render"
+            exit 1
+          fi
+
+      - name: Upload preview artifacts
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: catalog-previews
+          path: docs/images/catalog/
+          if-no-files-found: ignore
+          retention-days: 30