Accept hexadecimal key format (hex) in addition to npub/nsec

[mostronatorcoder]

Problem

Currently Mostrix only accepts bech32-encoded Nostr keys:

• Add Solver (Settings) → only accepts npub1... format
• Setup Admin Key → only accepts nsec1... format

However, operators often have hex-encoded keys from:

• rana output
• Mostro config files (config.toml, config.sample.toml)
• Raw Nostr tooling

Having to manually convert hex → bech32 to use Mostrix adds friction and causes errors.

Proposal

Accept both formats in all key input fields:

• npub1... (bech32) — current behavior
• nsec1... (bech32) — current behavior
• 64-char hex string — new

Scope

Two main input paths:

1. Add Solver (AdminMode::AddSolver)

   • Input: solver public key (hex or npub)
   • Validation: validate_npub() → try PublicKey::from_bech32(), fall back to PublicKey::from_hex()
   • Wire format: normalize to npub before sending AdminAddSolver to Mostro (so the node always receives bech32)

2. Setup Admin Key (AdminMode::SetupAdminKey)

   • Input: admin private key (hex or nsec)
   • Validation: validate_nsec() → try SecretKey::from_bech32(), fall back to SecretKey::from_str() (or equivalent hex parser)

UX

• Update placeholders in the input popup:
  • Add Solver: "npub... / hex..."
  • Setup Admin Key: "nsec... / hex..."
• Error messages should guide users: "expected npub1... (bech32) or 64-char hex string"

Context

This complements Mostro node's solver permission levels (PR #708), making it easier for operators to register AI agents (read-only) and human solvers (read-write) through Mostrix.