feat: show external plugins on the website (#937)

* feat: show external plugins on the website

Read plugins/external.json during website data generation and include
external plugins alongside local ones in plugins.json. External plugins
are flagged with external:true and carry metadata (author, repository,
homepage, license, source).

On the website:
- Plugin cards show a '🔗 External' badge and author attribution
- The 'Repository' button links to the source path within the repo
- The modal shows metadata (author, repo, homepage, license) and a
  'View Repository' CTA instead of an items list
- External plugins are searchable and filterable by tags

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address PR #937 security and UX review comments

- Add sanitizeUrl() function to validate URLs and prevent XSS via javascript:/data: schemes
- Add rel="noopener noreferrer" to all target="_blank" links to prevent reverse-tabnabbing
- Change external plugin path from external/<name> to plugins/<name> for proper deep-linking
- Track actual count of external plugins added (after filtering/deduplication) in build logs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/styles/global.css

@@ -900,6 +900,71 @@ body:has(#main-content) {
   color: var(--color-error);
 }
 
+/* External plugin badge */
+.resource-tag-external {
+  background: var(--color-accent);
+  color: var(--color-bg);
+  font-weight: 500;
+}
+
+/* External plugin modal metadata */
+.external-plugin-metadata {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  margin-bottom: 20px;
+  padding: 12px 16px;
+  background: var(--color-bg-tertiary);
+  border-radius: var(--border-radius);
+  border: 1px solid var(--color-border);
+}
+
+.external-plugin-meta-row {
+  display: flex;
+  align-items: baseline;
+  gap: 12px;
+  font-size: 13px;
+  line-height: 1.5;
+}
+
+.external-plugin-meta-label {
+  color: var(--color-text-muted);
+  font-weight: 500;
+  min-width: 80px;
+  flex-shrink: 0;
+}
+
+.external-plugin-meta-value {
+  color: var(--color-text);
+  word-break: break-all;
+}
+
+.external-plugin-meta-value a {
+  color: var(--color-accent);
+  text-decoration: none;
+}
+
+.external-plugin-meta-value a:hover {
+  text-decoration: underline;
+}
+
+.external-plugin-cta {
+  margin-bottom: 16px;
+}
+
+.external-plugin-repo-btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+}
+
+.external-plugin-note {
+  font-size: 13px;
+  color: var(--color-text-muted);
+  font-style: italic;
+  line-height: 1.5;
+}
+
 /* Page Layouts */
 .page-header {
   padding: 56px 0 40px;