Link support codes to agent sessions

- Server: Accept support_code param in WebSocket connection
- Server: Link code to session when agent connects, mark as connected
- Server: Mark code as completed when agent disconnects
- Agent: Accept support code from command line argument
- Agent: Send hostname and support_code in WebSocket params
- Portal: Trigger agent download with code in filename
- Portal: Show code reminder in download instructions
- Dashboard: Add machines list fetching (Access tab)
- Add TODO.md for feature tracking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/Cargo.toml

@@ -48,6 +48,9 @@ chrono = { version = "0.4", features = ["serde"] }
 # Hostname
 hostname = "0.4"
 
+# URL encoding
+urlencoding = "2"
+
 [target.'cfg(windows)'.dependencies]
 # Windows APIs for screen capture and input
 windows = { version = "0.58", features = [