Use linkify-it for commit description URLs

Replace the hand-rolled URL regex with `linkify-it` (already a
transitive dep via `marked-linkify-it`). Add unit tests.

package-lock.json

@@ -20,6 +20,7 @@
         "fuzzysort": "^3.1.0",
         "hast-util-to-dom": "^4.0.1",
         "hast-util-to-html": "^9.0.5",
+        "linkify-it": "^5.0.0",
         "lodash": "^4.18.1",
         "lru-cache": "^11.3.5",
         "marked": "^18.0.2",
@@ -38,6 +39,7 @@
         "@sveltejs/vite-plugin-svelte": "^7.0.0",
         "@tsconfig/svelte": "^5.0.8",
         "@types/katex": "^0.16.8",
+        "@types/linkify-it": "^5.0.0",
         "@types/lodash": "^4.17.24",
         "@types/md5": "^2.3.6",
         "@types/node": "^24",