fix: add manifest.json, link it in help.html, add tests

- manifest.json already present; add <link rel="manifest"> to help.html
  so all Nexus HTML pages declare PWA support consistently
- add tests/test_manifest.py to verify manifest exists, is valid JSON,
  has required PWA fields, and is referenced from both index.html and help.html

Fixes #832

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

help.html

@@ -12,6 +12,7 @@
 <link rel="preconnect" href="https://fonts.googleapis.com">
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
 <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@300;400;500;600&family=Orbitron:wght@400;600;700&display=swap" rel="stylesheet">
+<link rel="manifest" href="./manifest.json">
 <style>
   :root {
     --color-bg:            #050510;