Map: show stations heard in the last N days, from the kept history #455

Later work on top of persistent history (F1.41, docs/features/F1.41-persistent-history.md, decision 6). Today the map, station list and packet list show only the stations in memory: those heard within RemoveAfter (2 hours by default). Once 7 days of packets are kept, the map could show more than that.

Use cases

• After an event: "Who was on the air during Saturday's net / the marathon / the search?" Look back over the weekend and see every station heard in the area, including ones that went quiet hours ago.
• Coverage over time: "Which stations does my IGate or TNC hear over a week?" See the reach of a digipeater or IGate from more than the last two hours, including stations heard only now and then (a weekly net, a hilltop that's only on in good conditions).
• A station that went quiet: "Was N0CALL-9 heard yesterday, and where was it last?" Today it disappears from the map two hours after its last packet.
• Rare and occasional stations: balloons, satellites passing over, travellers going through the area, seen after the fact.

Proposal (outline, to be specced)

• A range control beside the view filters (F1.9): Live (today's behaviour, the default) · 24 h · 3 d · 7 d, up to the history's length. Per viewer.
• Older stations are added from the history, at their last kept position, drawn dimmed like stale stations (F1.3) with their age. A station still in memory is drawn as now. Selecting a dimmed station fills Details from the history (F1.41 behaviors 8 and 9).
• The station list and the packet list follow the range, with "from history" marked. The view filters still apply.
• Bounded: at most N stations from history in view (proposed: 10,000, the most recent first), loaded for the visible map area as it pans and zooms. A note says when the cap is reached.
• Read-only: stations from history never trigger alerts, messages, notifications or dashboard counts.
• Service surface: an IEngineService member returning the history's stations in a bounding box and time range, so the desktop and web apps behave the same.

Open questions

• A time slider ("what the map looked like at 14:00 Saturday"), or only "heard since"? The slider is a bigger feature: it means replaying positions, not just showing the last one.
• Whether tracks for every station in range are drawn, or only the selected station's (F1.41 behavior 8), as many tracks over 7 days could be heavy on a phone.

Depends on the F1.41 store.