feat(map): link upgrades — focus-on-hover, recency fade, hover card

[SimmerV]

Summary

• Recency fade — each link bakes a recencyOpacity factor from the most recent last_rx_time (NeighborInfo, both directions; falls back to last_seen for traceroute-inferred edges). Bands: <15 min full strength → 1 h taper to 0.6 → 6 h taper to 0.3.
• Rich hover card — maplibregl.Popup on all three link layers shows A ↔ B, kind (Neighbor / Heard by / Mutual / Traceroute), SNR, and relative last-heard. Follows cursor on mousemove.
• Unified link color — SNR gradient when known, gray when unknown. Drops the old kind-color fallback (green/blue/magenta), which read as "great link" when we just had no SNR data. Kind is now conveyed by line style only; traceroute keeps orange (per-hop SNR isn't meaningful on inferred paths).
• Legend rebuilt to match — SNR gradient bar, gray "unknown", line-style icons per kind, neutral white recency-fade swatch.

Deferred: SNR sparkline inside the hover card. Needs a backend endpoint that returns per-link SNR history (frontend fetches packets per node today, not per edge).