fix(textinput): resolve placeholder truncation on initial render

[sebastianhuus]

Summary

• Fixes placeholder text being truncated to first character on initial render
• Resolves issue where placeholder displays correctly only after width is calculated

Problem

The placeholderView function in textinput.go was creating a rune slice with size m.Width+1. When m.Width is 0 during initial render, this created a slice of length 1, truncating multi-character placeholders to just their first character.

Solution

• Changed slice allocation from m.Width+1 to max(len(m.Placeholder), m.Width+1) to ensure it can always hold the full placeholder
• Updated early return condition to check len(m.Placeholder) instead of len(p)

Test plan

• Verified fix resolves the truncation issue with gum input commands
• Tested with long placeholder: "Enter your full name and email address here"
• Tested with medium placeholder: "Your message here"
• Tested with short placeholder: "Name"
• Confirmed existing functionality remains intact

All tests show full placeholder text immediately on render, no more truncation.

Fixes #779

[sebastianhuus]

I also tested some various length placeholders. They all rendered properly with the caret highlighted on the first character and allowing input without a duplicate placeholder line.

Pre fix behavior:

Post fix behavior: