feat(website): surface subcategory labels as filterable tags

Entries nested under a plain-text subcategory heading (e.g. "Frameworks"
inside Testing) now carry a subcategory field populated by the parser.
The build pipeline collects these into a subcategories list on each merged
entry, and the template renders them as tag-subcat buttons that plug into
the existing data-cats filter mechanism.

A dedicated .tag-subcat style distinguishes them visually from category
tags, and both are hidden on mobile alongside .tag-group.

Co-Authored-By: Claude <noreply@anthropic.com>

website/build.py

@@ -102,6 +102,9 @@ def extract_entries(
                     existing["categories"].append(cat["name"])
                 if group_name not in existing["groups"]:
                     existing["groups"].append(group_name)
+                subcat = entry["subcategory"]
+                if subcat and subcat not in existing["subcategories"]:
+                    existing["subcategories"].append(subcat)
             else:
                 merged = {
                     "name": entry["name"],
@@ -109,6 +112,7 @@ def extract_entries(
                     "description": entry["description"],
                     "categories": [cat["name"]],
                     "groups": [group_name],
+                    "subcategories": [entry["subcategory"]] if entry["subcategory"] else [],
                     "stars": None,
                     "owner": None,
                     "last_commit_at": None,