AIS enhancements

#88 and #90 added AIS support. Here are some possible improvements to the AIS implementation.

• Track history — Accumulate recent positions per vessel, render as dashed trail line behind each target. Douglas-Peucker simplification to limit point count. Toggle via settings.
• Dangerous targets — Red dashed line on map between own vessel and targets with dangerously close CPA.
• AIS vessel list screen — Scrollable list of all visible AIS targets showing name, type, distance, bearing. Tap to center map on vessel and open detail sheet. Accessible from MainSheet.
• Flag/bookmark vessels — Mark specific vessels of interest. Flagged vessels get a visual indicator on the map and appear at the top of the AIS list. Persist via AsyncStorage.
• Wind vectors — Show apparent/true wind direction as lines from vessels that report wind data. Green for apparent, yellow for true.
• Aircraft & SAR contexts — Subscribe to aircraft.* and sar.* Signal K contexts. Render with distinct icons.
• Configurable staleness timeout — User setting for how long before targets go stale/expire (currently hardcoded). Options from 1 min to 24 hours per freeboard-sk.
• Vessel detail: dimensions diagram — Visual representation of vessel length, beam, and draft in the detail sheet.

Start by reading the AIS implementation from #88 and #90 and the MainSheet entry point mentioned for the vessel list. Choose and scope one enhancement, then verify the relevant map, settings, or detail-sheet behavior matches its stated outcome.