feat: Space key toggles FK preview popover

[datlechin]

Summary

• Add Space as the default shortcut to toggle FK preview popover on the focused cell (Quick Look-style)
• Registered as previewFKReference in the keyboard shortcut settings system — rebindable in Settings > Keyboard
• Replaces the hardcoded Cmd+Return FK preview with a settings-driven dispatch

Closes #648

Test plan

• Focus a FK column cell in the data grid, press Space — preview popover opens
• Press Space again — popover closes (toggle)
• Focus a non-FK column cell, press Space — nothing happens (no error)
• Open Settings > Keyboard, find "Preview FK Reference" under Edit — shows ␣ as default
• Rebind to a different shortcut (e.g., Cmd+Shift+P) — new shortcut works, Space stops working
• Clear the shortcut — neither Space nor any key triggers FK preview
• Reset to Defaults — Space works again