Step 1: Onion address parsing and validation

[igor53627]

Parent Issue

Part of #23 (Onion service support)

Overview

Add onion address parsing using tor-hscrypto crate.

Tasks

• Add tor-hscrypto dependency to webtor
• Create webtor/src/onion.rs module
• Implement OnionAddress type wrapping HsId
• Add helper to detect .onion hostnames in URLs
• Add unit tests for v3 onion address parsing
• Verify WASM build still works

Technical Notes

• tor-hscrypto already compiles for WASM
• HsId implements FromStr for parsing "${base32}.onion" addresses
• Validates checksum and version byte (v3 = 0x03)

Acceptance Criteria

• Can parse valid v3 onion addresses
• Rejects invalid/v2 addresses with clear errors
• All tests pass on both native and WASM