feat(sidebar): add favorite databases

[sophiathedev]

Closes #1553

Summary

• add database favorites from the Tree sidebar with Development, Testing, Production, and Unassigned environments
• show favorite databases first in the Favorites sidebar, grouped by environment with search, filtering, persisted disclosure, keyboard actions, and double-click switching
• keep favorites local and connection-scoped, clean them up when a connection is deleted, and document the behavior

Preview

Verification

• generated the Xcode project successfully
• passed an unsigned Debug build of the TablePro scheme
• passed 85 affected storage, grouping, menu, selection, AppKit, connection cleanup, and persistence tests in the deep pass
• passed the focused FavoritesOutlineSelectionTests again after the final persistence regression was added
• passed strict SwiftLint on all 34 changed Swift files with 0 violations
• verified the native flow against a disposable MySQL server in OrbStack: all groups render, the environment filter works, state survives relaunch, and double-click switches the active database

UI automation is covered at the AppKit coordinator boundary because the database Tree layout requires a live database server. The manual native pass used an isolated app sandbox and disposable server data.

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