feat: add PWA — mobile multi-model chat app (Vite + React)

- Vite + React + TypeScript + Tailwind v4
- vite-plugin-pwa for service worker and manifest
- Dark mode, borderless design with glow effects
- Reuses Elec components, adapted for mobile viewport
- Standalone-ready: can install to phone home screen

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

PWA/src/index.css

@@ -0,0 +1,36 @@
+@import "tailwindcss";
+
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+html, body, #root {
+  height: 100%;
+  background: #000;
+  color: #e5e5e5;
+  font-family: -apple-system, BlinkMacSystemFont, "SF Pro Text", "PingFang SC", sans-serif;
+  -webkit-font-smoothing: antialiased;
+  overflow: hidden;
+  user-select: none;
+}
+
+.glow-area {
+  position: relative;
+}
+.glow-area::before {
+  content: '';
+  position: absolute;
+  inset: 0;
+  background: radial-gradient(ellipse 65% 80% at 50% 50%, rgba(255,255,255,0.035) 0%, transparent 70%);
+  pointer-events: none;
+  z-index: 0;
+}
+
+.no-scrollbar::-webkit-scrollbar {
+  display: none;
+}
+.no-scrollbar {
+  scrollbar-width: none;
+}