Add privacy zones to exclude start and beginning of rides automatically

It is a bit annoying to crop the start and end with the slider. Especially when the rides are longer.

I would like to have the ability to define privacy zones as follows:

• Privacy zones can be defined by setting a point on the map and defining a radius (e.g. 100 m)
• When a ride is started or ends within a zone it is cropped automatically at the edge of the zone
• For privacy reasons some randomness should be added to the center and diameter of the zone each time a track is cropped based on the zone. Otherwise it is easy to determine the center of the zone.

That would enhance the privacy section of SimRa and covers use cases which the current delay settings can not be used for (e.g. privacy zone in the middle of a ride). Could also solve #13.

Start by reviewing the existing privacy section and delay settings, then read the related issue #13. Define how users set map points and radii, how rides are cropped at zone edges, and how per-crop randomness protects the zone center. Done means the requested privacy-zone behavior covers starts, ends, and zones in the middle of rides.