Mobile: add a level (1–5) filter

[Innovate-With-Sanjeev]

The mobile view (m.html) has cluster chips + search; add a level (1–5) filter too (Foundations → Advanced). Small and self-contained — great first issue.

Where: all in m.html.

• Levels come from LEVELS in js/data.js.
• Add a compact second row of chips like the existing .m-chips.
• Extend the matches(n) function to also check the active level.

Acceptance: level chips toggle; filtering combines with cluster + search; works offline; npm run build passes.

[Innovate-With-Sanjeev]

Shipped — the mobile view now has a level (1–5) filter row that combines with the cluster chips and search.