fix: position popovers using table view geometry instead of cell view bounds

[datlechin]

Summary

• Fix enum dropdown (and all other cell popovers) appearing at wrong position instead of near the clicked cell
• Replace cellView.bounds/cellView anchoring with tableView.rect(ofRow:).intersection(tableView.rect(ofColumn:)) anchoring to tableView
• Cell views can be recycled/relaid-out during click processing, making their bounds unreliable for popover positioning

Closes #209

Test plan

• Click an enum column cell — popover should appear directly below/above the cell
• Click a SET column cell — same positioning behavior
• Double-click a date column — date picker popover near the cell
• Double-click a FK column — FK popover near the cell
• Double-click a JSON column — JSON editor near the cell
• Test with scrolled table (rows not at top) — popover still anchored correctly

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