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/GlobalState.hpp

@@ -25,14 +25,14 @@ struct GlobalState {
           "plugin:hyprchrome:follow_hyprland_border_color",
           "Whether the border tracks Hyprland's own active/inactive border color instead of active_color/inactive_color",
           true),
-        .activeColor = makeShared<ColorValue>(
+        .activeColor = makeShared<GradientValue>(
           "plugin:hyprchrome:active_color",
-          "Color of the border on the focused window, used when follow_hyprland_border_color is false",
-          RGBAToARGB(0xFFD063FF)),
-        .inactiveColor = makeShared<ColorValue>(
+          "Color (or gradient) of the border on the focused window, used when follow_hyprland_border_color is false",
+          CHyprColor{RGBAToARGB(0xFFD063FF)}),
+        .inactiveColor = makeShared<GradientValue>(
           "plugin:hyprchrome:inactive_color",
-          "Color of the border on unfocused windows, used when follow_hyprland_border_color is false",
-          RGBAToARGB(0x6C6C6CFF)),
+          "Color (or gradient) of the border on unfocused windows, used when follow_hyprland_border_color is false",
+          CHyprColor{RGBAToARGB(0x6C6C6CFF)}),
         .titlebarHeight = makeShared<IntValue>(
           "plugin:hyprchrome:titlebar_height",
           "Height of the title bar hanging off the floating top-border cutout, in pixels",