Login webflow v3 to support simple signup and global scale

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Is your feature request related to a problem? Please describe.
As briefly discussed with @skjnldsv using login flow v2 is not possible, as the Desktop client does not know to which provider it should connect.

Describe the solution you'd like
So we need a "v3" with a gateway

• Desktop starts simple sign up in browser with generated token
  • pings nc.com/signup for continuous monitoring (same like v2)
• User chooses provider on web site, creates account, etc.
• after successful creation of the account the desired provider send nc:// info to Desktop via /signup (identified via token)

Additionally this needs to be backported to at 23/22/21 since else Apple M1 clients will need to rely on latest server version or 23+ which will likely cause issues in support at some point.

Motivation:

• QT for Apple M1 currently does not support QT WebEngine
• there might be proxy settings where it is better to use standard browser for this signup/login
• getting rid of v1 code on Desktop, as it is only used for preferred providers

Describe alternatives you've considered
none.

Additional context
First step is for the files-team to align the API with the clients-team or Tobias and Desktop-dev(s) respectively

Start by reviewing the existing login flow v2 and the proposed nc.com/signup gateway, including the token-based /signup handoff to Desktop. Coordinate the API between the files team and clients team; done means the v3 signup flow works across providers and is backportable to versions 21, 22, and 23.