fix: restore default is-outlined button visibility

[q121212]

Fixes #3996

Problem

Button with class is-outlined but without color modifier (like is-primary) renders invisible — white text on transparent background. Regression from v0.9.4.

Root Cause

$button-l defaults to scheme-main-l (100% = white in light theme). The default filled button does NOT use button-l — it uses button-background-l for bg, button-color-l for text, button-border-l for border. But is-outlined incorrectly uses button-l for BOTH text and border.

Fix

Change $button-l default from scheme-main-l to text-strong-l (1 character in variable name). Safe because:

• Default filled button doesn't use button-l
• Color modifiers override ALL variables
• Loading spinner color also becomes visible