feat: add connection groups

[imhuytq]

Summary

Adds a hierarchical connection groups system to the welcome window, letting users organize database connections into named, color-coded folders with arbitrary nesting depth.

Data model

• ConnectionGroup: id, name, color, parentGroupId (nesting), sortOrder (sibling ordering). Codable with migration support for pre-existing data.
• DatabaseConnection.groupId: optional link to a group.
• GroupStorage: CRUD, hierarchy helpers (childGroups, nextSortOrder, collectDescendantIds), and persisted expand/collapse state via UserDefaults.

Welcome window (ConnectionOutlineView)

• NSOutlineView-based tree: root groups → child groups → connections, sorted by sortOrder.
• Group cells show a color-tinted folder icon and recursive connection count.
• Expand/collapse: double-click or Return key; state persisted across app launches.
• Drag & drop: move connections into/out of groups; reorder connections and groups within a parent; move groups to become children of other groups. Circular nesting is prevented at drop validation.
• Context menus:
  • On group: New Connection, New Subgroup, Edit Group, Delete Group.
  • On connection: Connect, Edit, Duplicate, Move to Group (nested submenu), Delete.
  • On empty space: New Connection, New Group.
• Search mode flattens the tree to a filtered flat list.

Group form sheet (ConnectionGroupFormSheet)

• Fields: name, color picker (8 colors), parent group picker.
• ParentGroupPicker: recursive nested menus; when editing, self and all descendants are excluded to prevent cycles.
• Newly created groups auto-expand in the outline view.

Connection form (ConnectionGroupEditor)

• Drop-in menu in the Appearance section showing the current group.
• Recursive nested submenus for group selection.
• Inline "Create New Group" and "Manage Groups" (delete) actions.

Deletion behavior

• Deleting a group also deletes all its descendants.
• Member connections are ungrouped (not deleted).

Test plan

• Create a root group and a nested subgroup; verify tree renders correctly
• Drag a connection into a group, then to a different group, then to root (ungrouped)
• Drag a group into another group; verify circular nesting is blocked
• Reorder connections and groups by drag within the same parent
• Edit a group: change name, color, and parent group
• Delete a group with nested subgroups; verify all descendants and connections are handled
• Expand/collapse groups; restart the app and verify state is restored
• Create a connection and assign a group from the connection form
• Search for a connection; verify the flat list shows correctly
• Run GroupStorageTests

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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'

[imhuytq]

@datlechin please review