Site map view #7

Motivation: Sites already have latitude and longitude. A
map view lets the user see where they dive and explore sites in
regions they're traveling to.

What to build:

• Sites window → View menu → Map (or a "Show on Map" button)
• Use QWebEngineView with a Leaflet map, or pyqtgraph with
  OpenStreetMap tiles
• Plot all sites as markers, color-coded by environment
• Click a marker → shows the site's name, country, max depth
• Optional: filter the map to current search
• Offline fallback: if no internet, show a static image with all
  the markers

Acceptance criteria:

• Map loads within 3s with 3,000+ sites
• Markers are clickable and show site info
• A test seeds 100 sites with random lat/lon and asserts the map
  renders

Related: #2 (search filter on the map), #15 (geolocation fill-in)