fix(docs-demo): sync admin settings sections with manifest
The demo Settings screen was fed by a frozen snapshot baked into the externally generated demoDataset.js, so it drifted from the real manifest: the Remnawave Panel (plus System and Migrations) sections were missing and trial/checkout/common still showed as top-level sections instead of subsections. Generate frontend/src/lib/webapp/settingsManifest.generated.json from manifest_payload() (the same source the live /admin/settings endpoint uses) via scripts/export_settings_manifest.py, and build the demo section structure from it, overlaying realistic demo values per field key. A pytest drift guard fails if the Python manifest changes without regenerating the snapshot, so the demo stays in sync going forward.
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