feat(sidebar): add database filter to the tree sidebar

[desperadoxhy]

What

The tree sidebar can now show only a chosen set of databases. With tree layout on, a funnel button appears at the bottom of the sidebar. Click it, toggle "Only show selected databases", and check the databases you want to see. Toggle off shows all.

Why

Servers often hold dozens of databases. The tree gets noisy when most are ones you never open. This hides them per connection.

How

• DatabaseTreeFilterStorage (UserDefaults, per connection, device-local) stores the enable toggle and the selected database set.
• DatabaseTreeView.visibleDatabases filters by the set when the toggle is on.
• SidebarView bottom bar gets a funnel button plus a popover with a toggle, a checkbox list, and Select All / Clear.
• Deleting a connection clears its filter (single and batch).

Tests

Unit tests for the storage cover defaults, persistence, round-trip, per-connection isolation, and single/batch cleanup.

Verification

This machine only has Command Line Tools (no full Xcode), so I could not run xcodebuild. I verified API signatures and call sites by static review. Please build and run TableProTests/DatabaseTreeFilterStorageTests on a machine with Xcode.

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[desperadoxhy]

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