fix: render textarea placeholder base style once

[leno23]

Summary

• make the textarea placeholder path return the raw viewport output so Base is applied only by View()
• keep placeholder rendering consistent with the normal text rendering path
• add a regression test that catches doubled borders when Base has a border

Test plan

• go test ./textarea
• go test ./...
• git diff --check

Fixes charmbracelet/bubbletea#1643