Add minimum sightings filter and AP estimation enhancements

[MatthieuGrr]

Thanks for this tool, it's exactly what I was looking for. Two things I added while using it on my own captures:

Min sightings per BSSID (Filters). A slider to hide networks seen fewer than N times,
to get rid of drive-by noise. Counts are based on the current selection, so they follow
the other filters.

Estimate every shown network at once (AP Locator). Same estimation as the existing
per-AP option, but applied to everything shown, one pin per BSSID. Clicking a pin still
drills into it like before.

A few notes:

• Both are off by default, nothing changes if you don't enable them.
• The batch mode uses all sightings of a BSSID, same as the existing single-AP view, so a
  pin doesn't move when you click into it. Estimates are cached, otherwise dragging a
  slider re-runs trilateration on every AP.
• In batch mode the exported lat/lon are estimates and not GPS fixes, so the file is named
  wardrive_estimated_aps.csv and has 4 extra columns. Normal export is unchanged.
• Small fix along the way: focusAP() was passing "" for records with no BSSID, which
  merged all of them into one bogus AP. Both call sites now use a shared bssidKey().

Single file, +140/-22, no new dependencies. Happy to split it in two PRs if you prefer.