feat: add public share page with download support (#253)

Share links now point to /s/{token} (was /api/s/{token}) and render a
proper HTML page instead of raw JSON.

Changes:
- static/share.html: standalone public share page
- static/css/views/share-public.css: share page styles
- static/js/views/public/publicShare.js: client-side logic that fetches
  share metadata via /api/s/{token}, handles password-protected shares,
  and renders file download / folder info
- build.rs: include share.html in the HTML embed pipeline
- web/mod.rs: serve /s/{token} route (unauthenticated)
- share_dto.rs: generate URLs as /s/{token} instead of /api/s/{token}
- share_handler.rs: new download_shared_file() handler that validates the
  share token and streams file content without requiring authentication
- routes.rs: mount GET /api/s/{token}/download (public, uses AppState)

static/css/views/share-public.css

@@ -0,0 +1,132 @@
+/* share-public.css — stand-alone styles for the public share page */
+* { box-sizing: border-box; margin: 0; padding: 0; }
+
+body {
+  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+  background: var(--color-bg-hover);
+  color: var(--color-text-heading);
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  min-height: 100vh;
+  padding: 1rem;
+}
+
+.share-card {
+  background: var(--color-bg-surface);
+  border-radius: var(--radius, 12px);
+  box-shadow: 0 4px 24px var(--color-shadow-sm);
+  padding: 2.5rem;
+  max-width: 480px;
+  width: 100%;
+  text-align: center;
+}
+
+.share-logo {
+  margin-bottom: 1.5rem;
+}
+.share-logo h1 {
+  font-size: 1.5rem;
+  font-weight: 700;
+}
+.share-logo span {
+  color: var(--color-primary);
+}
+
+/* States */
+.share-state { }
+.hidden { display: none !important; }
+
+h2 {
+  font-size: 1.15rem;
+  margin-bottom: 0.5rem;
+}
+p.subtitle {
+  color: var(--color-text-gray);
+  font-size: 0.9rem;
+  margin-bottom: 1.25rem;
+}
+
+/* Spinner */
+.spinner {
+  width: 36px;
+  height: 36px;
+  border: 3px solid var(--color-border);
+  border-top-color: var(--color-primary);
+  border-radius: 50%;
+  margin: 0 auto 1rem;
+  animation: spin 0.7s linear infinite;
+}
+@keyframes spin { to { transform: rotate(360deg); } }
+
+/* Icons */
+.share-icon {
+  font-size: 3rem;
+  margin-bottom: 0.75rem;
+}
+
+/* Password form */
+#password-form {
+  text-align: left;
+}
+#password-input {
+  width: 100%;
+  padding: 0.75rem 1rem;
+  font-size: 1rem;
+  border: 2px solid var(--color-border);
+  border-radius: 8px;
+  outline: none;
+  background: var(--color-bg-surface);
+  color: var(--color-text-heading);
+  transition: border-color 0.2s;
+  margin-bottom: 0.75rem;
+}
+#password-input:focus {
+  border-color: var(--color-primary);
+}
+.error-text {
+  color: var(--color-error, #e53935);
+  font-size: 0.85rem;
+  margin-bottom: 0.75rem;
+}
+
+/* Primary button */
+.btn-primary {
+  display: inline-block;
+  width: 100%;
+  padding: 0.75rem 1.5rem;
+  background: var(--color-primary);
+  color: #fff;
+  border: none;
+  border-radius: 8px;
+  font-size: 0.95rem;
+  font-weight: 600;
+  cursor: pointer;
+  text-decoration: none;
+  text-align: center;
+  transition: background 0.2s;
+}
+.btn-primary:hover {
+  opacity: 0.9;
+}
+
+/* File view */
+#file-name {
+  word-break: break-word;
+}
+#file-meta {
+  font-size: 0.85rem;
+}
+
+/* Folder view */
+.folder-list {
+  text-align: left;
+  margin-top: 1rem;
+  max-height: 400px;
+  overflow-y: auto;
+}
+
+/* Responsive */
+@media (max-width: 500px) {
+  .share-card { padding: 1.5rem; }
+}