Cards: optional repo link, block-level descriptions, schema + CI validation

- Project card: optional `repo` field renders a git icon corner link
  (github.com / bitbucket.org / Gitea URLs supported).
- All cards: `.card__title` and `.card__desc` are now display:block, so
  the description always starts on its own line.
- Added data/links.schema.json (JSON Schema 2020-12) describing the
  full content model. Both data files reference it via `$schema` for
  github.dev / VS Code autocomplete.
- New GitHub Action validates data/links.json on every push/PR:
  jq syntax check + ajv-cli schema validation.

.github/workflows/validate-data.yml

@@ -0,0 +1,39 @@
+name: Validate links data
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "data/links.json"
+      - "data/links.example.json"
+      - "data/links.schema.json"
+      - ".github/workflows/validate-data.yml"
+  pull_request:
+    paths:
+      - "data/links.json"
+      - "data/links.example.json"
+      - "data/links.schema.json"
+      - ".github/workflows/validate-data.yml"
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: JSON syntax (jq)
+        run: |
+          set -e
+          for f in data/links.json data/links.example.json data/links.schema.json; do
+            echo "Checking $f"
+            jq empty "$f"
+          done
+
+      - name: Schema validation (ajv)
+        run: |
+          npx --yes ajv-cli@5 validate \
+            --spec=draft2020 \
+            --strict=false \
+            -s data/links.schema.json \
+            -d data/links.json \
+            -d data/links.example.json