feat(seo): add robots.txt, sitemap, and noindex tags for downloads

Register routes for robots.txt and sitemap.xml, and implement search engine indexing controls to protect user privacy.

Specifically:
- Set `X-Robots-Tag: noindex, nofollow, noarchive` headers on file downloads, thumbnails, and zip generation.
- Configure `Robots: web.RobotsNone` on download and preview pages to prevent indexing of temporary user uploads.
- Add canonical URLs, improved descriptions, and image alt tags to page metadata for better social sharing.

backend/libs/handlers/pages.go

@@ -60,9 +60,12 @@ func (a *App) Home(w http.ResponseWriter, r *http.Request) {
 	maxUploadSize, limitSummary := a.homeUploadPolicyLabels(settings, user, loggedIn, isAdmin)
 	expiryOptions, defaultExpiry := a.homeExpiryOptions(settings, user, loggedIn, isAdmin)
 	a.renderPage(w, r, http.StatusOK, "home.html", web.PageData{
-		Title:       "Upload your files",
-		Description: "Upload and share files through a self-hosted Warpbox instance.",
-		CurrentUser: currentUser,
+		Title:        "Upload your files",
+		Description:  "Upload and share files fast. Drop a file, get a link — private, temporary transfers that expire on your terms.",
+		CanonicalURL: absoluteURL(r, "/"),
+		ImageURL:     absoluteURL(r, "/static/og-default.png"),
+		ImageAlt:     "Warp Box — simple file sharing and fast downloads",
+		CurrentUser:  currentUser,
 		Data: homeData{
 			MaxUploadSize:        maxUploadSize,
 			LimitSummary:         limitSummary,