User location handling

From my POV, there's three main issues with the current user location handling:

1. Currently, the map repeatedly centers onto the user location when "Continuously" is selected.
This is rather distracting and should be avoided - a first "centering" might be acceptable, but a deliberate "zoom to my location"-button would be more fitting (and also more in line with the crosshair-design of the icon).

2. The "only once"-functionality should still have a "focus on my location"-button. Currently, when moving the map after using the "Only once"-functionality, the location button changes back to the "Show my location"-functionality

3. The crosshairs design of the "stop showing my location"-icon is a bit misleading. Something along the lines of [Location disabled](https://fonts.google.com/icons?icon.query=location&selected=Material+Symbols+Outlined:location_disabled:FILL@0;wght@400;GRAD@0;opsz@48) might be more to the point.

No files or tests are named. Start by tracing user-location controls and map-centering behavior in the VueJS, Vuetify, and Vue2Leaflet client; done means continuous mode does not repeatedly recenter, only-once retains a focus action, and the disabled-location icon is clearer.

Written by the indexing model from the issue text.