feat: support responsive Grid layout (conditional minWidth)

[cassdeckard]

Summary

Support responsive Grid layout where the same primitive can be placed in different grid positions depending on screen size, by adding the same item multiple times with different minWidth/minHeight thresholds. tview supports this via multiple AddItem calls; tviewyaml currently adds each grid item once.

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The Grid slide adds menu/main/sidebar with minWidth: 0 for narrow screens (hidden) and minWidth: 100 for wide screens (visible), achieving responsive layout.

Demo Usage

Demo adds same primitives multiple times with different minWidth (0 vs 100) for narrow/wide; tviewyaml adds each item once.

Proposed Approach

• Extend GridItem to support multiple placements: placements: [{ row, column, rowSpan, colSpan, minHeight, minWidth, focus }, ...]
• Or add responsiveItems that duplicate a primitive with different min dimensions
• In populateGridItems, call AddItem for each placement

Relevant Files

• config/types.go – extend GridItem or add responsive structure
• builder/builder.go – populateGridItems
• docs/tview-coverage.md – document once implemented