feat: add expand button for truncated cell values in result tables

[aalvaropc]

Summary

• Cells with text >500 chars now show an expand icon (↗) next to the truncated value
• Clicking the expand icon opens a modal with the full, untruncated cell content
• Modal uses monospace font with word-wrap for readability
• Non-truncated cells are unaffected — click-to-copy still works as before
• New view_cell / close_cell_view events and viewing_cell assign

Test plan

• mix precommit — 631 tests, 0 failures
• Manual: run a query returning long text (e.g., SELECT repeat('x', 1000)), verify expand button appears and modal shows full text