Upgrade dependencies to latest stable versions, migrate Tailwind CSS v3 to v4

Bumped React, Supabase JS, and type packages to latest patch/minor releases.
Migrated the Tailwind pipeline from the v3 JS-config format to v4's CSS-first
@theme setup (@tailwindcss/postcss, @import "tailwindcss" in globals.css),
removing tailwind.config.ts and autoprefixer since v4 handles both natively.
All existing color/radius/font tokens were ported 1:1 so no component classes
needed to change. TypeScript and ESLint were bumped and tested against their
true latest majors (7.x and 10.x) but both broke the build/lint pipeline, so
they're pinned to the latest working minor instead (TypeScript 5.9.3, ESLint
9.39.5).

app/globals.css

@@ -1,8 +1,45 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
+
+/* Design tokens from spec §7 (ported from tailwind.config.ts for Tailwind v4) */
+@theme {
+  --color-brand-50: #fdf3f8;
+  --color-brand-100: #fdeaf3;
+  --color-brand-200: #f6cfe2;
+  --color-brand-500: #d6046e;
+  --color-brand-600: #b0035a;
+  --color-brand-700: #a30354;
+
+  --color-surface: #f9f1f5;
+
+  --color-border: #eedde6;
+  --color-border-subtle: #f7e8ef;
+
+  --color-ink: #2d1c26;
+  --color-ink-body: #503b47;
+  --color-ink-muted: #7a636f;
+
+  --color-success-bg: #dff6dd;
+  --color-success-text: #0e700e;
+
+  --color-danger-bg: #fde7e9;
+  --color-danger-text: #b10e1c;
+  --color-danger-solid: #c50f1f;
+
+  --color-warning-bg: #fff4ce;
+  --color-warning-text: #835b00;
+
+  --color-info-bg: #d7f0f0;
+  --color-info-text: #00666d;
+
+  --color-purple-bg: #f0ecf7;
+  --color-purple-text: #5c2e91;
+
+  --radius: 8px;
+
+  --font-sans: var(--font-nunito), system-ui, sans-serif;
+}
 
 body {
-  background-color: theme("colors.surface");
-  color: theme("colors.ink.DEFAULT");
+  background-color: var(--color-surface);
+  color: var(--color-ink);
 }