feat(datagrid): read-only cell viewer with unified interaction dispatcher

[datlechin]

Summary

• Double-click or press Return on a query result cell now opens an inline NSTextView overlay (read-only, selectable, copyable). JSON columns open the JSON viewer in a popover, BLOB columns open the hex viewer.
• Adds CellInteractionResolver (pure value-type): single source of truth that maps cell context to a CellInteractionMode. Replaces the duplicated decision trees previously scattered across handleDoubleClick, insertNewline, and handleChevronAction.
• Extracts CellOverlayBase shared base class for CellOverlayEditor and CellOverlayViewer — positioning, dismiss observers, raise-to-front. Editor down 295→120 LOC, viewer 235→70 LOC, base 200 LOC.
• JSONViewerView(isEditable: false) reused for read-only JSON popover via new JSONViewerContentView wrapper.
• HexEditorContentView gets isEditable parameter; viewer mode hides the editable hex section and Save button.

Fixes part 2 of #1336 (part 1 — Cmd+C single-cell — landed via #1337/#1338).

Test plan

• Open a query result (SELECT 'A', 'B'). Double-click A cell — inline overlay opens with A selected. Cmd+C copies just A.
• Cmd+A in the viewer then Cmd+C → copies full cell value.
• Esc, click outside, scroll, or column resize — viewer dismisses.
• Press Return on a focused read-only cell — viewer opens.
• JSON column cell in read-only mode → JSON viewer popover opens with syntax highlighting + Tree mode.
• BLOB column cell in read-only mode → hex viewer popover opens (no editable hex section, no Save).
• Editable browse table: double-click still routes to inline edit / overlay editor / JSON editor / BLOB editor / FK popover as before.
• Run resolver tests: xcodebuild test -only-testing:TableProTests/CellInteractionResolverReadOnlyTests + CellInteractionResolverEditableTests.

Architecture

The refactor introduces three new value-types and one new sibling overlay class, eliminating ~140 LOC of geometry+observer duplication. Cell-interaction routing is now driven by one resolver; double-click, Return key, and (future) chevron action all consume the same CellInteractionMode.