Favicon, level file icons, and a feed error mark in the triangle's column

- The logo as favicon, squared up (it is 128x121), at /favicon.png and at
  /favicon.ico outside the auth layer, where a browser asking on its own got a
  401; an apple-touch-icon on white (#32).
- An item not yet downloaded had its download bar on a line of its own under the
  file icon, lifting the icon above its row's; the bar now sits under it without
  taking space (#31).
- A feed error is Font Awesome's exclamation, hung in the margin where a folder's
  triangle is, in the same column; a folder holding a failing feed has its
  triangle turn red.

Closes #31, #32.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/web.rs

@@ -65,6 +65,9 @@ pub fn router(state: WebState) -> Router {
         .route("/login", get(login_page))
         .route("/api/login", post(login))
         .route("/icon.png", get(icon))
+        .route("/favicon.ico", get(favicon))
+        .route("/favicon.png", get(favicon))
+        .route("/apple-touch-icon.png", get(touch_icon))
         .route("/app.js", get(app_js))
         .route("/login.js", get(login_js))
         .route("/inter.woff2", get(inter))
@@ -477,6 +480,25 @@ async fn icon() -> impl IntoResponse {
     )
 }
 
+/// The logo, squared up with transparent padding: it is 128x121, and a tab icon that is not
+/// square can be passed over. /favicon.ico is the same PNG, for a browser that asks for that
+/// on its own; behind the auth layer it answered 401, and the tab stayed blank.
+async fn favicon() -> impl IntoResponse {
+    (
+        [(header::CONTENT_TYPE, "image/png"), (header::CACHE_CONTROL, "max-age=86400")],
+        include_bytes!("../web/favicon.png").as_slice(),
+    )
+}
+
+/// For an iPhone's home screen, which paints a transparent icon's background black, so this
+/// one is on white.
+async fn touch_icon() -> impl IntoResponse {
+    (
+        [(header::CONTENT_TYPE, "image/png"), (header::CACHE_CONTROL, "max-age=86400")],
+        include_bytes!("../web/apple-touch-icon.png").as_slice(),
+    )
+}
+
 /// Inter, the pages' typeface, served from the binary as the icon is, so neither page loads
 /// anything from anyone else. Outside the auth layer for the sign-in page. Its licence, the SIL
 /// Open Font License, is web/Inter-LICENSE.txt.