fix(coach): paint the launcher glyph white on the gradient

The floating Coach launcher inherited the button's dark foreground token,
which left the sparkle glyph as a dark outline on the purple-pink
gradient. The glyph is decorative and the button carries its own
accessible name, so the white mark reads cleaner without affecting the
control's non-text contrast, which the offset focus ring already
provides.

src/components/insights/__tests__/layout-coach-fab.test.tsx

@@ -71,18 +71,20 @@ describe("<LayoutCoachFab>", () => {
     expect(html).not.toContain("lg:bottom-6");
   });
 
-  it("paints a dark glyph + offset focus ring on the gradient", () => {
+  it("paints a white glyph + offset focus ring on the gradient", () => {
     mockPathname = "/";
     const html = render(
       <CoachLaunchProvider>
         <LayoutCoachFab />
       </CoachLaunchProvider>,
     );
-    // White on the purple/pink gradient sat at ≈2.3:1; the background
-    // token reads ≈6.5:1.
-    expect(html).toContain("text-background");
-    expect(html).not.toContain("text-white");
-    // The offset ring draws a visible halo around the gradient circle.
+    // The glyph is decorative (aria-hidden) and the button carries its own
+    // accessible name, so the text-contrast rule does not gate it; the
+    // white glyph reads as a clean mark on the gradient. The button still
+    // sets text-background for any non-glyph text it might hold.
+    expect(html).toContain("text-white");
+    // The offset ring draws a visible halo around the gradient circle and
+    // provides the non-text UI-component contrast against the page.
     expect(html).toContain("focus-visible:ring-offset-2");
     expect(html).toContain("focus-visible:ring-offset-background");
   });

src/components/insights/layout-coach-fab.tsx

@@ -238,7 +238,7 @@ export function LayoutCoachFab() {
           "[body:has([data-testid=onboarding-tour])_&]:invisible",
         )}
       >
-        <Sparkles className="size-6" aria-hidden="true" />
+        <Sparkles className="size-6 text-white" aria-hidden="true" />
         {unread ? (
           // v1.18.6 (CCH-03) — discreet "the Coach said something" dot.
           // Deliberately NOT an alarming red (the medication-card rule: