feat: add retry with exponential backoff and progress feedback (#12)

[rafabd1]

Feature: Add retry with exponential backoff and progress feedback (#12)

Closes #12

Problem

When I2P tunnels are degraded or the peer is temporarily unavailable, initiate_session fails immediately with a generic error. This results in:

1. Poor user experience: Users see an error immediately and must manually retry
2. No progress feedback: Users don't know if the app is working or stuck
3. No automatic recovery: Temporary network issues cause permanent failure
4. Uninformative errors: Generic messages don't help users understand what's happening

This is particularly problematic on I2P, where tunnel establishment can take 30-60s and tunnels often degrade temporarily.

Solution

Implement retry with exponential backoff and progress feedback:

1. Retry loop with exponential backoff:

   • Maximum 5 attempts before giving up
   • Backoff delays: 2s, 4s, 8s, 16s between retries
   • Generates new ephemeral keys for each attempt (forward security)

2. Progress events:

   • connection_progress: Emitted before each attempt with attempt, max_attempts, and status
   • Allows frontend to show progress bar or status to user

3. Error events:

   • connection_error: Emitted after each failed attempt with descriptive message
   • Frontend can display errors to help users understand what's happening

4. Timeouts:

   • 60s timeout for ACK response (configurable)
   • Prevents indefinite hanging on unresponsive peers

5. Clear final error:

   • After all retries, emit final error explaining the situation
   • Suggests possible causes (peer offline, tunnel degraded)

Implementation details

• Retry attempts wrap the entire handshake process (connection + send init + receive ack)
• Each retry generates new ephemeral keys for perfect forward security
• Exponential backoff with 2^n seconds (capped at 16s)
• All operations wrapped in proper error handling to ensure retries work correctly
• Events use standard Tauri event system for frontend integration