Mobile PWA: remember the last-used filter chip across sessions

[Innovate-With-Sanjeev]

The task

On the mobile PWA (m.html), remember the visitor's last-used filter chip (Core / a use case / All) across sessions, so reopening the app restores where they were.

Why

The mobile app already persists theme (kuber-theme) and My Map (kuber-mymap). Persisting the active filter is a small, self-contained UX win and a friendly first contribution.

Where to start

• m.html only (desktop app.html is a separate UI — don't touch it).
• The active filter is the active variable; chips are built in buildChips() and applied in matches()/render().
• Mirror the kuber-theme localStorage pattern. Suggested key: kuber-filter. Fall back to core when absent or stale.

Done when

• Selecting a chip writes it to localStorage; reload restores it.
• Invalid/stale values fall back to core (no crash).
• Desktop UI unchanged; bump the service-worker.js cache version.