feat: Add resizable columns to the workflows table.

[tristal]

Description & motivation 💭

The workflows table has no way to control column widths. Columns autosize to their content, and the only lever is the dense/comfortable toggle.

This adds drag-to-resize on the workflows table header:

• Drag the divider at the right edge of any column header to set its width.
• Widths persist per-namespace in the existing column config (localStorage), alongside label/order/pinned.
• Double-click a divider to return that column to autosizing. A reset button appears next to the density toggle once any column is resized, to clear all widths.
• Keyboard accessible: tab to a divider, arrow keys resize in 16px steps, Enter/Space resets. Minimum width 80px.

Screenshots (if applicable) 📸

Design Considerations 🎨

• Dividers are invisible at rest so the table looks unchanged; hovering the header row reveals them all as subtle 1px lines (discoverability), and the divider under the cursor thickens to 2px in the accent color.

Testing 🧪

How was this tested 👻

• Manual testing
• E2E tests added
• Unit tests added

Docs

Any docs updates needed?

[vercel]

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