fix: guard against follow list overwrite when local list is empty

[barrydeen]

Summary

• When a user with an empty local follow list tries to follow someone, we now query relays for an existing kind-3 event before allowing the follow to proceed
• A confirmation dialog appears immediately (blocking the follow) showing a spinner while the relay check runs (up to 5s)
• If a follow list is found on relays, it's loaded and the follow is added silently — no user action needed
• If no list is found, the dialog updates to warn the user that following will start a new list at 1, and advises rebroadcasting from another client if they believe this is wrong
• After logout with no remaining accounts, navigation now goes to the splash screen instead of the login screen

Test plan

• Log in with an account that has no follow list on relays, attempt to follow from the feed (long-press avatar) — confirm dialog appears with spinner, then confirmation message after ~5s
• Log in with an account that has a follow list but it hasn't loaded yet — attempt to follow — confirm dialog auto-dismisses and follow is added once relays respond
• Log in with an account that has a follow list already loaded — follow normally with no dialog
• Log out with no other accounts — confirm splash screen appears, not login screen
• Onboarding follow-all flow is unaffected (no dialog)

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