refactor: migrate to vue3

[zameji]

Migrated to Vue3 (as Vue2 is EoL for a while now). Did also major changes to the UI:

• better support for horizontal screens
• more compact feature selection
• individual features may be renamed to keep track of them while searching
• pagination with prefetching the next batch in the background
• results may be removed from the list (allowing discarding candidates)
• better performance on low-end computers even for larger queries

Backend changes

• Return geometries (addresses #5)
• Handle multiple instances of same feature correctly (i.e. not returning duplicates)

There are still some TODOs open:

• Modularize the stores
• The OneTap-Login was dropped for now