Implement multi-step Add Connection modal with Ark UI Steps

[mkobayashime]

Summary

• Split Add Connection modal into a 2-step wizard flow using Ark UI Steps component
• Extract reusable FolderTree component from FolderSelectPopup for better code organization
• Improve UX by separating repository setup from target folder selection

Changes

Step 1: Repository Setup

• Repository selection using existing RepoCombobox
• Source directory (srcDir) configuration
• Validation before proceeding to step 2

Step 2: Target Folder Selection

• Inline folder tree with expand/collapse functionality
• Refresh button to reload bookmark folders
• Visual selection indicator

Components

• FolderTree.tsx (new): Reusable folder tree component with hierarchical display
• FolderSelectPopup.tsx: Refactored to use FolderTree, reducing code duplication
• AddConnectionModal.tsx: Complete rewrite with Ark UI Steps integration

UI Improvements

• Visual step indicator with numbered circles and labels
• Grid-based modal layout with fixed height (480px)
• Scrollable folder tree to prevent overflow
• Linear step flow enforced through validation
• Consistent styling with existing design patterns

Test plan

• Verify step 1 → step 2 navigation with valid inputs
• Verify step 1 validation prevents proceeding without repository selection
• Verify step 2 → step 1 back navigation
• Verify inline FolderTree expand/collapse functionality
• Verify Complete button validates target folder selection
• Verify Cancel button closes modal from any step
• Verify Refresh button reloads folder tree
• Verify folder tree scrolls when content exceeds modal height
• Verify styling matches existing modal patterns