Render border gradients instead of a flat first stop

The frame collapsed Hyprland's border gradient to m_colors.front(), so a
gradient col.active_border showed up as a single flat color.

Snapshot the gradient (stops + angle) into a new header-only ChromeGradient
and fill the cairo path with a linear gradient built from it:

- AxisFor() reproduces the border shader's quadrant-folding progress
  function (y*sin(a) + x*(1-sin(a)), not a true rotation) as a cairo axis,
  so the frame's sweep stays in step with the window border it wraps.
  Checked against a port of the shader across all 360 degrees: cardinal
  angles exact, worst case 0.003 of the sweep (the shader folds on literal
  1.57/3.14/4.71 where this uses pi).
- SampleAt() interpolates in OkLab, where the shader interpolates, with
  each segment subdivided into sampled cairo stops so cairo's own sRGB lerp
  tracks that curve.

active_color/inactive_color become gradient config values, taking the same
syntax as general:col.active_border. Single-color configs are unaffected.

The cross-focus fade (m_realBorderColorPrevious + fade progress, lerped
between two gradients in-shader) is still not reproduced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ChromeConfig.hpp

@@ -13,8 +13,10 @@ struct ChromeConfig {
   // inactiveColor below.
   SP<BoolValue> followHyprlandBorderColor;
 
-  SP<ColorValue> activeColor;
-  SP<ColorValue> inactiveColor;
+  // Gradients (multiple stops + an optional angle), same syntax as
+  // Hyprland's general:col.active_border/col.inactive_border.
+  SP<GradientValue> activeColor;
+  SP<GradientValue> inactiveColor;
 
   // Height of the title bar hanging off the floating top-border cutout, in
   // logical pixels.