fix(a11y): give the due-reminder action a readable foreground

The Vorsorge "Jetzt messen" / mark-done button hard-coded white text on a
solid green fill. In the dark theme the fill is the bright Dracula green, so
white-on-green reads about 1.37:1 — below the WCAG AA floor.

Add a theme-aware `--success-foreground` token (near-black glyph on the dark
fill, white on the darker light-mode green) mirroring the existing
`--primary-foreground` pattern, and use `text-success-foreground` on both the
card and list variants. Green stays only on the action button; the card
surface remains neutral.

src/app/globals.css

@@ -96,6 +96,7 @@
    *   - info    → dracula-cyan    (neutral informational note)
    */
   --color-success: var(--success);
+  --color-success-foreground: var(--success-foreground);
   --color-warning: var(--warning);
   --color-info: var(--info);
 
@@ -155,6 +156,17 @@
   --warning: var(--dracula-orange);
   --info: var(--dracula-cyan);
 
+  /*
+   * Foreground for glyphs/labels painted on a solid `bg-success` fill. The
+   * raw Dracula green (`#50fa7b`) is a bright tone, so white-on-green reads
+   * ~1.37:1 in dark mode — well under AA. The dark theme below pairs it with
+   * the near-black Dracula background (`#282a36`, ~10.4:1); the light theme
+   * keeps white on the darker light-mode green (`#14720a`, ~6.1:1). Mirrors
+   * the `--primary-foreground` pattern. Default here keeps the dark-theme
+   * value so an un-themed root never falls back to an unreadable pairing.
+   */
+  --success-foreground: #282a36;
+
   --radius: 0.625rem;
 }
 
@@ -212,6 +224,9 @@
   --status-pill-near: #ffb86c;
   --status-pill-out: #ff8a8a;
 
+  /* Dark glyph on the bright Dracula-green fill — ~10.4:1 on `#50fa7b`. */
+  --success-foreground: #282a36;
+
   /*
    * Dose accent in dark mode — the exact Tailwind `text-purple-400`
    * (`oklch(0.714 0.203 305.504)`) the cards used before, so the dark card
@@ -297,6 +312,8 @@
   --success: #14720a;
   --warning: #a34d14;
   --info: #036a96;
+  /* White glyph on the darker light-mode green — ~6.1:1 on `#14720a`. */
+  --success-foreground: #ffffff;
   --border: #cfcfde;
   --input: #cfcfde;
   --ring: #644ac9;

src/components/measurement-reminders/vorsorge-section.tsx

@@ -753,9 +753,9 @@ function VorsorgeCard({
         : "";
   const isCoach = reminder.origin === "COACH";
   const isLinked = reminder.measurementType != null;
-  // Due now / overdue ⇒ the action button takes the green "do it now" tone,
-  // analogous to the medication card's due affordance. The CARD stays neutral
-  // (no tint) per the project rule — only the action button goes green.
+  // Due now / overdue ⇒ the action button takes the green "do it now" tone.
+  // The CARD stays neutral (no tint) per the project rule — only the action
+  // button goes green; the surface follows the medication-card grammar.
   const isDue =
     due.key === "nextDue.today" || due.key === "overdueByDays";
   const progress = intervalProgress(reminder, now);
@@ -817,13 +817,15 @@ function VorsorgeCard({
 
   // v1.18.6 (MOD-06) — the green "Jetzt messen" / mark-done action. Green only
   // on the action button (never the card surface). When not due it stays the
-  // calm default tone so the green reads as "now is the time".
+  // calm default tone so the green reads as "now is the time". `bg-success`
+  // pairs with the theme-aware `text-success-foreground` (dark glyph in dark,
+  // white in light) so the label clears WCAG AA in both themes.
   const primaryButton = (
     <Button
       type="button"
       className={cn(
         "min-h-11 w-full",
-        isDue && "bg-success text-white hover:bg-success/90",
+        isDue && "bg-success text-success-foreground hover:bg-success/90",
       )}
       onClick={onPrimaryAction}
       disabled={busy}
@@ -889,7 +891,7 @@ function VorsorgeCard({
                 size="sm"
                 className={cn(
                   "min-h-9",
-                  isDue && "bg-success text-white hover:bg-success/90",
+                  isDue && "bg-success text-success-foreground hover:bg-success/90",
                 )}
                 onClick={onPrimaryAction}
                 disabled={busy}