feat: allow multi-character corners on borders

[toller892]

Description

Remove the single-rune truncation on border corners (TopLeft, TopRight, BottomLeft, BottomRight).

Currently, applyBorder truncates all corner strings to a single rune (lines 406-410 in borders.go):

// For now, limit corners to one rune.
border.TopLeft = getFirstRuneAsString(border.TopLeft)
border.TopRight = getFirstRuneAsString(border.TopRight)
border.BottomRight = getFirstRuneAsString(border.BottomRight)
border.BottomLeft = getFirstRuneAsString(border.BottomLeft)

This prevents users from using multi-character corner decorations. However, the renderHorizontalEdge function already uses ansi.StringWidth to correctly handle multi-character left/right parts, so the truncation is unnecessary.

What changed

• Removed the 4-line truncation block in applyBorder
• Replaced with a comment explaining corners may be multi-character
• Added tests verifying multi-character corners work correctly

Example

With this change, you can create borders like:

┌──                 ── ┐
│                      │
│                      │
└──                 ── ┘

Using a custom border:

customBorder := lipgloss.Border{
    Top:         "-",
    Bottom:      "-",
    Left:        "|",
    Right:       "|",
    TopLeft:     "──",
    TopRight:    "──",
    BottomLeft:  "──",
    BottomRight: "──",
}

Fixes #605