Add R Shiny app for NWFSC Survey Grid SST visualization

[Copilot]

Implements an interactive Shiny app that fetches daily SST from NOAA ERDDAP and overlays values on NWFSC Combo survey grid points via a Leaflet map.

app.R

• Reads available date range from ERDDAP metadata (ncdcOisst21Agg_LonPM180) to bound the dateInput picker
• Fetches SST via rerddap::griddap() clipped to the grid bounding box; matches values to grid points with a vectorised nearest-neighbour join
• Grid filtered to survey == "NWFSC.Combo" from surveyjoin::nwfsc_grid; each point assigned a stable cell_id
• Points coloured with viridis palette via leaflet::colorNumeric; legend added via addLegend()
• Popups expose cell_id and SST (°C)
• Fetch is eventReactive on button click — no spurious re-fetches on date change alone
• Both get_erddap_dates() and fetch_sst() wrapped in tryCatch; missing/unavailable dates surface a sidebar message rather than crashing the session

README.md

• Replaces placeholder content with installation steps (system libs + R packages), run instructions (console and CLI), and a usage guide