landing: structured feature groups, single-source comparison table (#11)

- The ~1,800-word fork paragraph becomes 7 titled groups, one short line
  per feature, plus a link to LOTUS_FEATURES.md.
- Single source: landing/data/features.json + comparison.json, rendered
  into index.html by landing/build.py between BEGIN/END markers (all 71
  table rows re-render with identical cell text).
- CSS moved to landing/style.css; build.py inlines it because the LXC 139
  deploy copies index.html only.
- "Client comparison last reviewed" stamp driven by comparison.json.
- CI job `landing-fresh` fails if index.html drifts from its sources.
- Feature list brought up to date: 629 avatar decorations in 28
  categories, mic level meter + remembered per-person volume, search
  operators, paste-as-code, thread "Mark all read", pinch-zoom/swipe media
  viewer, room widgets with a permission prompt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

.gitea/workflows/lint.yml

@@ -46,6 +46,18 @@ jobs:
             exit 1
           fi
 
+  landing-fresh:
+    name: Landing page is rendered (matrix #11)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      # index.html's feature groups, comparison table and inlined CSS are
+      # generated from landing/data/*.json + landing/style.css. Fail if someone
+      # edited the data without re-running the renderer (or edited the output).
+      - name: Check landing/index.html matches its sources
+        run: python3 landing/build.py --check
+
   python-lint:
     name: Python (ruff)
     runs-on: ubuntu-latest