Text labels on canvas don't adapt to dark/light theme switch

When switching between light and dark themes, text labels placed on the canvas (via the text tool) keep their original color and don't adapt to the new theme.

Root cause: Each text label stores its own ATTR_COLOR at creation time. The TEXT_TOOL_COLOR preference only affects newly created labels — existing labels are unaffected by applyThemeColors()

Why this isn't a simple fix: Automatically inverting all existing label colors on theme switch would overwrite user-chosen colors, which is a design decision (feature), not a bug. Options to consider:

1. Only auto-invert labels that used the default color
2. Add a per-label "auto" color option that follows the theme
3. Add a global "auto-adapt text color" preference

Discovered in:

• #2655

And here is the label in dark theme