feat: add npub read-only login

[barrydeen]

Summary

• Allow login with npub or 64-char hex pubkey for read-only browsing — signer is null so all write actions silently no-op
• Add READ_ONLY signing mode to KeyRepository with savePubkeyReadOnly() and isReadOnly()
• Refactor AuthViewModel.logIn() to detect input type (nsec/npub/hex) and route accordingly
• Read-only login skips existing-user onboarding and goes straight to loading screen
• Fix pre-existing bug: clear stale notifications, events, and DMs in reloadForNewAccount() so account switches don't show old data

Test plan

• Login with an npub → skips onboarding → lands on feed with that user's content
• Login with 64-char hex pubkey → same behavior
• Tap reply/react/repost/compose → nothing happens (silent no-op)
• Navigate to profiles, notifications, relays → all viewable
• DMs tab → conversations empty (can't decrypt)
• Keys screen → shows npub only
• Logout → back to auth screen
• Login with nsec → full functionality still works
• Login with remote signer → still works
• Logout of one account, login to another → notifications show new account's data