Replace PicoCSS with DaisyUI + Tailwind v4

Use gotailwind (standalone Tailwind v4 via Go tool) with DaisyUI
plugin files — no npm needed. CSS is compiled at build time and
embedded via a Via Plugin that serves it as a static file.

Custom "connect4" theme: light, warm, playful palette with red/yellow
accents matching game pieces and board blue accent.

assets/css/input.css

@@ -0,0 +1,63 @@
+@import 'tailwindcss';
+
+@source not "./daisyui{,*}.mjs";
+@plugin "./daisyui.mjs";
+@plugin "./daisyui-theme.mjs" {
+  name: "connect4";
+  default: true;
+  color-scheme: light;
+
+  /* Playful & bright — warm, casual board game feel */
+  --color-base-100: oklch(98% 0.005 90);       /* warm white */
+  --color-base-200: oklch(95% 0.01 90);        /* light cream */
+  --color-base-300: oklch(90% 0.015 85);       /* warm gray border */
+  --color-base-content: oklch(25% 0.02 50);    /* dark warm text */
+
+  --color-primary: oklch(60% 0.22 30);         /* warm red — matches game pieces */
+  --color-primary-content: oklch(98% 0.005 90);
+  --color-secondary: oklch(82% 0.17 85);       /* golden yellow — matches game pieces */
+  --color-secondary-content: oklch(25% 0.02 50);
+  --color-accent: oklch(65% 0.19 250);         /* board blue */
+  --color-accent-content: oklch(98% 0.005 90);
+  --color-neutral: oklch(35% 0.03 260);
+  --color-neutral-content: oklch(95% 0.01 90);
+
+  --color-success: oklch(72% 0.19 155);
+  --color-success-content: oklch(20% 0.04 155);
+  --color-warning: oklch(82% 0.17 85);
+  --color-warning-content: oklch(25% 0.04 85);
+  --color-error: oklch(60% 0.22 30);
+  --color-error-content: oklch(97% 0.01 30);
+  --color-info: oklch(70% 0.15 240);
+  --color-info-content: oklch(20% 0.04 240);
+
+  --radius-selector: 0.5rem;
+  --radius-field: 0.5rem;
+  --radius-box: 0.75rem;
+  --border: 1px;
+  --depth: 1;
+  --noise: 0;
+};
+
+/* Game-specific styles that can't be Tailwind utilities */
+.board {
+  display: flex;
+  gap: 8px;
+  background: #2563eb;
+  padding: 16px;
+  border-radius: 12px;
+}
+.column { display: flex; flex-direction: column; gap: 8px; padding: 4px; border-radius: 8px; }
+.column.clickable { cursor: pointer; }
+.column.clickable:hover { background: rgba(255,255,255,0.15); }
+.cell { width: 48px; height: 48px; border-radius: 50%; background: #1e40af; transition: background 0.2s; }
+.cell.red { background: #dc2626; }
+.cell.yellow { background: #facc15; }
+.cell.winning { animation: pulse 0.5s ease-in-out infinite alternate; }
+@keyframes pulse {
+  from { transform: scale(1); box-shadow: 0 0 10px rgba(255,255,255,0.5); }
+  to { transform: scale(1.1); box-shadow: 0 0 20px rgba(255,255,255,0.8); }
+}
+.player-chip { width: 20px; height: 20px; border-radius: 50%; background: #666; }
+.player-chip.red { background: #dc2626; }
+.player-chip.yellow { background: #facc15; }