Reset Add Connection modal state on close

[mkobayashime]

Summary

• Fixed modal state persistence issue by using conditional rendering to unmount the component when closed
• Eliminates stale data (repo selection, srcDir, target folder, step position, errors) when users reopen the modal
• Simpler and more maintainable than manual state reset logic

Changes

• Wrapped AddConnectionModal in conditional rendering ({addOpen && ...})
• Component now unmounts completely when closed, automatically resetting all internal state
• Added documentation file explaining the approach

Benefits

• Automatic state reset: No manual cleanup needed
• React idiomatic: Uses component lifecycle for cleanup
• Future-proof: Any new state additions automatically reset
• Clean UX: Modal always starts fresh when reopened

Test plan

• Open modal, fill out fields, click Cancel → reopen → verify all fields reset
• Complete full flow to add connection → reopen modal → verify clean state
• Trigger validation error → close → reopen → verify no error persists
• Partially fill form (change srcDir) → cancel → reopen → verify srcDir back to "/"