cleanup(legibility): repo hygiene — untrack artifacts, update gitignore

CLEANUP-4 (#415):

Untracked from git (files stay on disk where appropriate):
- frontend/e2e/.auth/user.json — dev credential, already gitignored in
  frontend/.gitignore; git rm --cached so the rule takes effect
- proofshot-artifacts/ (44 files, ~7.6MB) — browser verification
  screenshots committed by mistake; added root .gitignore entry
- frontend/.svelte-kit.old/ — stale type stub from stammbaum route
  rename; deleted from disk
- frontend/test-results.locked/ — Playwright E2E artifacts; deleted
  from disk
- node_modules/.vite/vitest/.../results.json — Vite test cache committed
  by mistake

Deleted from repo:
- package.json / package-lock.json at root (3 testing-library devDeps
  with no justification for living outside frontend/)

.gitignore additions:
- root: proofshot-artifacts/, node_modules/
- frontend: **/test-results.locked/, **/.svelte-kit.old/

After this commit, git status on a fresh clone shows zero unexpected
items (only docs/superpowers/ and familienarchiv-408/ remain untracked,
both pre-existing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/e2e/.auth/user.json

@@ -1,25 +0,0 @@
-{
-  "cookies": [
-    {
-      "name": "PARAGLIDE_LOCALE",
-      "value": "de",
-      "domain": "localhost",
-      "path": "/",
-      "expires": 1812352142.362504,
-      "httpOnly": false,
-      "secure": false,
-      "sameSite": "Lax"
-    },
-    {
-      "name": "auth_token",
-      "value": "Basic%20YWRtaW5AZmFtaWx5YXJjaGl2ZS5sb2NhbDphZG1pbjEyMw%3D%3D",
-      "domain": "localhost",
-      "path": "/",
-      "expires": 1777878542.943668,
-      "httpOnly": true,
-      "secure": false,
-      "sameSite": "Strict"
-    }
-  ],
-  "origins": []
-}