UAT backlog: additional scenarios for differentiating features

[perdrizat]

The UAT tier (tests/uat/, see tests/uat/README.md and TESTING.md "UAT tests") currently covers 11 scenarios: env/smoke (00, 01), tiles (10, 11), drawer (20–23), and design (30–32). Several differentiating features that benefit from visual/judgment-based review have no dedicated scenario yet.

Scenarios to add

• Locked-grid. Visual confirmation that a locked grid genuinely blocks drag-reorder feedback (no drop-preview affordances render) while everything else stays interactive.
• Per-domain filter caps. A capped domain's excess tiles are visibly absent/replaced, including the subdomain-wildcard (.example.com) case.
• Per-tile background colour. Custom tile background colours render correctly against both light and dark themes, including contrast with overlaid title text.
• Backup export. The exported backup file's contents are sane (spot-check via the restore flow) and the UI gives clear success feedback.
• Multi-page grids. Grids large enough to paginate render page-navigation controls correctly and preserve tile identity across pages.

Notes

• Each new scenario is a plain-English .md file per the existing numbering convention (extend the 1x/2x category ranges as appropriate).
• Author only scenarios where structural DOM tests cannot prove aesthetic correctness or absence of occlusion — purely functional behavior stays at the E2E tier (see TESTING.md "When to write a scenario").
• UAT is pre-release/local-only and never gates CI — these scenarios are for pre-release runs and manual review only.