feat: add SetBorderPadding for Box-like primitives

[cassdeckard]

Summary

Add borderPadding (top, right, bottom, left) configuration for Box-like primitives (TextView, Table, etc.) so padding inside the border can be set via YAML. tview provides SetBorderPadding.

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The Table and Code TextView slides use SetBorderPadding(1, 1, 2, 0) and similar for layout.

Demo Usage

Table and Code TextView use SetBorderPadding; not in Primitive config.

Proposed Approach

• Add borderPadding: [top, right, bottom, left] to Primitive (or shorthand for uniform padding)
• Apply in ApplyProperties for any primitive that implements SetBorderPadding (Box interface)
• Support both 4-element list and single-value uniform padding

Relevant Files

• config/types.go – add borderPadding
• builder/properties.go – apply to Box-like primitives
• docs/tview-coverage.md – document once implemented