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/ChromeDecoration.hpp

@@ -3,6 +3,7 @@
 #define WLR_USE_UNSTABLE
 
 #include "Globals.hpp"
+#include "ChromeGradient.hpp"
 #include <hyprland/src/desktop/DesktopTypes.hpp>
 #include <hyprland/src/render/decorations/IHyprWindowDecoration.hpp>
 #include <hyprutils/math/Box.hpp>
@@ -42,17 +43,19 @@ public:
   // by `chamferPx`, plus a title bar of height `titleBarHeightPx` and width
   // `titleBarWidthPx` (already measured/clamped by the caller - see
   // ChromeDecorationGeometry) hanging off the floating top-border
-  // cutout. Cached and only regenerated when any of these change from the
-  // last call.
-  SP<Render::ITexture> GetBorderTexture(const Vector2D& sizePx, float extentPx, float chamferPx, uint64_t colorValue, float titleBarHeightPx, float titleBarWidthPx);
+  // cutout. Filled with `gradient` swept across the whole texture along that
+  // gradient's own axis (a single-stop gradient is just a flat fill). Cached
+  // and only regenerated when any of these change from the last call.
+  SP<Render::ITexture> GetBorderTexture(const Vector2D& sizePx, float extentPx, float chamferPx, const ChromeGradient& gradient, float titleBarHeightPx, float titleBarWidthPx);
 
   // Returns a texture of `title` rendered via Hyprland's own text renderer
   // (Pango, through IHyprRenderer::renderText) at `textSizePx` using
   // `fontFamily`, truncated with a trailing "…" if it doesn't fit within
-  // `maxWidthPx` - black while `isActive`, 0xEEEEEEFF otherwise, both at the
-  // border color's own alpha. Cached and only regenerated when any of these
-  // change from the last call. Returns nullptr if `title` is empty.
-  SP<Render::ITexture> GetTitleTexture(const std::string& title, float textSizePx, uint64_t colorValue, const std::string& fontFamily, int maxWidthPx, bool isActive);
+  // `maxWidthPx` - black while `isActive`, 0xEEEEEE otherwise, both at
+  // `alpha` (the border's own alpha, so translucent borders keep translucent
+  // text). Cached and only regenerated when any of these change from the last
+  // call. Returns nullptr if `title` is empty.
+  SP<Render::ITexture> GetTitleTexture(const std::string& title, float textSizePx, float alpha, const std::string& fontFamily, int maxWidthPx, bool isActive);
 
   WP<ChromeDecoration> self;
 
@@ -64,14 +67,14 @@ private:
   Vector2D cachedTexSize = {-1, -1};
   float cachedExtent = -1.F;
   float cachedChamfer = -1.F;
-  uint64_t cachedColorValue = 0;
+  ChromeGradient cachedGradient;
   float cachedTitleBarHeight = -1.F;
   float cachedTitleBarWidth = -1.F;
 
   SP<Render::ITexture> cachedTitleTex;
   std::string cachedTitleTexTitle;
   float cachedTitleTexSize = -1.F;
-  uint64_t cachedTitleTexColor = 0;
+  float cachedTitleTexAlpha = -1.F;
   std::string cachedTitleTexFont;
   int cachedTitleTexMaxWidth = -1;
   bool cachedTitleTexActive = false;