feat(web): componentize frontend and replace native delete confirm

- Split monolithic app.js/style.css into Alpine.js components (icons, store, fileSection, confirm)
- Add central Alpine.store('files') as single source of truth for file lists and actions
- Replace duplicated Inbox/Outbox markup with reusable fileSection component (props: dir, canShare)
- Add shared window.Icons module to remove duplicated inline SVGs
- Replace browser-native confirm() for file deletion with a styled, accessible modal dialog
- Split CSS into design tokens, base, and component layers; modernize visuals
- Update web.go embed and server static routes for the new component/style dirs

web/styles/base.css

@@ -0,0 +1,54 @@
+*,
+*::before,
+*::after {
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+}
+
+html {
+  -webkit-text-size-adjust: 100%;
+}
+
+body {
+  font-family: var(--font-sans);
+  background: var(--color-bg);
+  background-image: var(--color-bg-gradient);
+  background-attachment: fixed;
+  color: var(--color-text);
+  line-height: 1.5;
+  min-height: 100vh;
+  -webkit-font-smoothing: antialiased;
+  text-rendering: optimizeLegibility;
+}
+
+button {
+  font-family: inherit;
+}
+
+[x-cloak] {
+  display: none !important;
+}
+
+a {
+  color: inherit;
+}
+
+:focus-visible {
+  outline: 2px solid var(--color-primary);
+  outline-offset: 2px;
+  border-radius: var(--radius-sm);
+}
+
+/* ===== App shell ===== */
+.app {
+  max-width: 760px;
+  margin: 0 auto;
+  padding: var(--space-8) var(--space-5) var(--space-16);
+}
+
+@media (max-width: 600px) {
+  .app {
+    padding: var(--space-5) var(--space-4) var(--space-16);
+  }
+}