feat: add connection groups

[imhuytq]

Summary

• Add ConnectionGroup model with color and nesting support
• Add GroupStorage for persisting connection groups
• Add ConnectionGroupEditor and ConnectionGroupFormSheet views
• Update ConnectionFormView, ConnectionStorage, DatabaseConnection, and WelcomeWindowView to support groups

Test plan

• Create a new connection group with a name and color
• Nest connections inside a group on the welcome screen
• Edit and delete a connection group
• Verify connections are correctly associated with groups after restart

[chatgpt-codex-connector]

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[datlechin]

Run bị lỗi rồi anh ơi

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift
/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:239:8 Type 'DatabaseConnection' does not conform to protocol 'Hashable'

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:239:8 Type 'DatabaseConnection' does not conform to protocol 'Equatable'

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:256:9 Invalid redeclaration of 'groupId'

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:276:9 Invalid redeclaration of 'groupId'

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:313:1 Type 'DatabaseConnection' does not conform to protocol 'Decodable'

/Users/ngoquocdat/Projects/TablePro/TablePro/Models/DatabaseConnection.swift:313:1 Type 'DatabaseConnection' does not conform to protocol 'Encodable'