auth.proto says the Hub generates client_id, but the clients generate it locally

[bibonix]

The comment on Auth.client_id in auth.proto says:

Not a UUID — the Hub generates it once per installation.

That last clause is wrong. The Hub does not generate client IDs; the clients do, locally, and the Hub just stores whatever id the caller sends.

• ios/APNCommon/LegalPrincipal.swift — LegalPrincipal.init() builds a 9-char string from ABCDEFGHIJKLMNPQRSTUVWXYZ123456789 using SystemRandomNumberGenerator.
• android/src/main/java/tech/apn/common/Principal.kt — Principal() does the same with kotlin.random.Random and the same alphabet, and SystemTunnels.kt persists it to prefs the first time.
• hub/objects/hub/clients.rb — Hub::Clients#auth(id, ...) just INSERT ... ON CONFLICT DO NOTHING with the id passed in; nothing on the Hub generates one.
• hub/front/front_auth.rb — POST /auth takes params[:uid] verbatim (only upcases it and rejects 000000000), so even the auth route accepts a caller-supplied id rather than minting one.

Suggested fix: change the last sentence of the client_id comment to something like "Not a UUID — the client generates it once per installation and persists it across launches."

[bibonix]

@yegor256 could you take a look at this?