BUG: Plugin execution blocks main thread

[andrinoff]

Describe the bug: Lua plugins execute synchronously on main thread. Long-running plugins freeze entire UI. No timeout or cancellation.

To reproduce:

1. Create plugin with slow HTTP call or infinite loop
2. Trigger plugin on email arrival
3. UI completely frozen
4. Cannot cancel or interrupt
5. Must kill process

Expected behavior:

• Execute plugins in goroutines
• Set execution timeout (5s default, configurable)
• Show "Plugin running..." indicator
• Allow user to cancel long-running plugins
• Kill plugin goroutine on timeout
• Log timeout errors

Additional context:

• File: plugin/plugin.go
• Need goroutine + timeout wrapper
• Context cancellation support
• Protect against malicious/buggy plugins
• Severity: Medium - UI freeze
• Complexity: Medium-High - concurrency + Lua state