feat(radar): rain radar tab with FMI observed + forecast overlay

[eetu]

New Sade tab: a Leaflet map centred on the saved location with pinch-zoom/pan, an animated timeline, and a recenter button.

Backend (radar module)

• GET /api/radar/frames — last-hour observed radar frames, derived from FMI WMS GetCapabilities (5-min steps).
• GET /api/radar/wms — transparent proxy for FMI WMS GetMap tiles (single fixed upstream, streams bytes).
• GET /api/radar/forecast — FMI HARMONIE precipitation forecast: fetches the EPSG:4326 GRIB2 grid, decodes it with the grib crate (default-features = false, no native C deps), reads grid geometry straight from the GDS (no PROJ), orients north-up, block-max downsamples, converts metres→mm, returns a compact JSON grid. Retries transient FMI connection drops; cached 30 min.

Frontend (RainMap, lazy-loaded)

• Observed WMS frames + forecast heat overlays (canvas colour ramp) merged into one timeline: play/pause, frame slider, a "now" divider, long-press-swipe scrub, relative-time labels (−30 min → nyt → +5 h), and a precipitation legend.
• Pinch-zoom + pan native; CARTO base tiles swap with the light/dark theme.

Notes

• FMI WMS/WFS need no API key. New settings FMI_WMS_BASE_URL / FMI_DOWNLOAD_BASE_URL default to the public endpoints.
• Forecast covers southern Finland (~500×380 km) centred on the saved location; whole-Finland would need tiling (single lat/lon overlay misregisters over a tall span).
• Forecast magnitudes cross-validated against FMI's own WFS point forecast.

Test plan

• cargo test — 20 unit + 18 integration pass (incl. GRIB orientation/resampling)
• yarn validate (typecheck + lint + format) clean; yarn build succeeds
• Endpoints verified live against FMI (frames, WMS tile, forecast grid)