chore: remove stale src/frontend (React/Vite) + cleanup .gitignore

Frontend was migrated to Qute/Alpine in 34e5246. The src/frontend/ tree
was no longer built, not referenced in any build file (pom.xml,
Dockerfile, .dockerignore), and not present on disk. Removing:

- 36 tracked-but-deleted React/TS/Vite/Tailwind files
- 5 untracked screenshot artifacts in repo root
- 4 stale entries in .gitignore (src/frontend/node_modules/, dist/, etc.)

.gitignore

@@ -2,10 +2,6 @@ target/
 .mvn/
 node/
 node_modules/
-src/frontend/node_modules/
-src/frontend/dist/
-src/frontend/tsconfig.app.tsbuildinfo
-src/frontend/tsconfig.node.tsbuildinfo
 /data/
 .idea/
 .vscode/