fix: destructive confirm buttons had unreadable black-on-red text

--destructive-foreground was never defined in globals.css (light or dark
theme) — text-destructive-foreground resolved to nothing, so both the
clear-chat-history confirm button and recipe-form's discard-changes
confirm (same bg-destructive text-destructive-foreground pattern) fell
back to the default dark text color on a solid red background.

Added the missing variable (white, matching --primary-foreground's
light value — both themes use a saturated enough red that white stays
legible), mapped through @theme inline alongside the existing
--color-destructive.

Verified locally: screenshotted the clear-history confirm dialog,
button now shows white text on red.

apps/web/app/globals.css

@@ -27,6 +27,7 @@
   --color-input: var(--input);
   --color-border: var(--border);
   --color-destructive: var(--destructive);
+  --color-destructive-foreground: var(--destructive-foreground);
   --color-accent-foreground: var(--accent-foreground);
   --color-accent: var(--accent);
   --color-muted-foreground: var(--muted-foreground);
@@ -64,6 +65,7 @@
   --accent: oklch(0.97 0 0);
   --accent-foreground: oklch(0.205 0 0);
   --destructive: oklch(0.577 0.245 27.325);
+  --destructive-foreground: oklch(0.985 0 0);
   --border: oklch(0.922 0 0);
   --input: oklch(0.922 0 0);
   --ring: oklch(0.708 0 0);
@@ -99,6 +101,7 @@
   --accent: oklch(0.269 0 0);
   --accent-foreground: oklch(0.985 0 0);
   --destructive: oklch(0.704 0.191 22.216);
+  --destructive-foreground: oklch(0.985 0 0);
   --border: oklch(1 0 0 / 10%);
   --input: oklch(1 0 0 / 15%);
   --ring: oklch(0.556 0 0);