Add R Shiny app for NWFSC Survey Grid SST visualization

[Copilot]

Implements the requested Shiny app that renders NWFSC.Combo survey grid points on a Leaflet map colored by daily SST from NOAA ERDDAP.

app.R

• Grid: loads surveyjoin::nwfsc_grid filtered to survey == "NWFSC.Combo", assigns sequential cell_id
• Date picker: bounds populated at startup from rerddap::info() actual_range metadata; falls back to 1981-09-01 → today-2 if ERDDAP is unreachable
• SST fetch: rerddap::griddap("ncdcOisst21Agg_LonPM180", ...) scoped to the grid bounding box; wrapped in eventReactive so it only fires on date change
• Spatial join: rounds both datasets to 0.25° (native OISST resolution) for a fast left_join — no heavy spatial dependency needed
• Map: colorNumeric(viridis::viridis(256)) circle markers + addLegend; click fires observeEvent showing Cell ID, lon/lat, and SST in both a Leaflet popup and a sidebar verbatimTextOutput

# Reactivity is scoped to date changes only
sst_data <- eventReactive(input$selected_date, {
  rerddap::griddap("ncdcOisst21Agg_LonPM180", url = erddap_url,
                   time = c(date_str, date_str),
                   longitude = c(lon_min, lon_max),
                   latitude  = c(lat_min, lat_max),
                   fields = "sst", fmt = "csv")
})

README.md

Replaces the placeholder with system prerequisites, package install commands (pak), and run instructions.

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