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;

tests/test_manifest.py

@@ -0,0 +1,39 @@
+"""Tests for manifest.json PWA support. Fixes #832 (Missing manifest.json)."""
+import json
+from pathlib import Path
+
+
+def test_manifest_exists() -> None:
+    assert Path("manifest.json").exists(), "manifest.json must exist for PWA support"
+
+
+def test_manifest_is_valid_json() -> None:
+    content = Path("manifest.json").read_text()
+    data = json.loads(content)
+    assert isinstance(data, dict)
+
+
+def test_manifest_has_required_pwa_fields() -> None:
+    data = json.loads(Path("manifest.json").read_text())
+    assert "name" in data, "manifest.json must have 'name'"
+    assert "short_name" in data, "manifest.json must have 'short_name'"
+    assert "start_url" in data, "manifest.json must have 'start_url'"
+    assert "display" in data, "manifest.json must have 'display'"
+    assert "icons" in data, "manifest.json must have 'icons'"
+
+
+def test_manifest_icons_non_empty() -> None:
+    data = json.loads(Path("manifest.json").read_text())
+    assert len(data["icons"]) > 0, "manifest.json must define at least one icon"
+
+
+def test_index_html_references_manifest() -> None:
+    content = Path("index.html").read_text()
+    assert 'rel="manifest"' in content, "index.html must have <link rel=\"manifest\">"
+    assert "manifest.json" in content, "index.html must reference manifest.json"
+
+
+def test_help_html_references_manifest() -> None:
+    content = Path("help.html").read_text()
+    assert 'rel="manifest"' in content, "help.html must have <link rel=\"manifest\">"
+    assert "manifest.json" in content, "help.html must reference manifest.json"