fix(login): keep the account session when logging in to a Personal Server

[volod-vana]

What broke

Two separate faults in vana login, both hit while chasing why vana connect
reported "Personal Server sync failed / requires authentication".

1. Logging in to a Personal Server signed you out of your account.
saveCredentials rewrites the whole auth file. The self-hosted branch handed
it a freshly built account block:

account: {
  address: result.address,   // the SERVER OWNER's wallet, not your account
  session_token: "",         // a live account token, blanked
  expires_at: result.expires_at,
},

So vana login --server <url> silently replaced the CLI identity with
whoever owns that server and destroyed a valid account session. On a machine
where the CLI account and the local server owner differ, that is a real
identity swap with no warning and no confirmation.

2. vana login could not fix the state it had just diagnosed.
The server URL is resolved only from env vars and auth.json. With
personal_server: null the URL is unknown, so the command took the cloud
branch, saw a valid account token, printed Already logged in, and returned
0 having done nothing. It then suggested vana login --server <url> for a URL
it had detected through a different code path, and immediately below that
told the user to run vana logout first. Every connect kept failing with
MISSING_AUTH and the two printed remedies contradicted each other.

What changed

• accountSessionToPreserve decides which account block survives a server
  login. A live session is carried across; a tokenless or env-sourced one is
  not persisted, so a first-time self-hosted user still gets seeded from the
  server owner as before.
• When the server's owner differs from the signed-in account, the CLI now
  says so instead of quietly switching.
• The already-logged-in path acquires the missing server session itself
  rather than printing the command back at the user. Gated on --no-input,
  and on the server actually being self-hosted, so agents and CI keep the old
  reporting behaviour.
• The contradictory logout first line now reads as what it is: how to sign
  in as someone else.

The self-hosted flow moved into loginToPersonalServer so both entry points
share it. That move is most of the diff; the logic inside is unchanged apart
from the credential merge.

Test plan

• pnpm validate (367 tests)
• New unit tests cover all three branches of accountSessionToPreserve
• Verified against a stub Personal Server in a throwaway HOME, before and
  after the fix. Before: account_address became the server owner and the
  token was emptied. After: account address and token intact, server
  session stored, mismatch reported.
• Verified the already-logged-in path with a detected-but-unauthenticated
  server: plain vana login now completes the server login and writes
  personal_server, where it previously exited 0 with nothing saved.
• Confirmed --no-input still takes the reporting path

[github-actions]

🎉 This PR is included in version 0.23.13 🎉

The release is available on GitHub release

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