fix(icons): true alpha channel, no white matte or white tile

- public/icon.png and src/assets/logo.png were grayscale (mode L): a flat
  white field, no alpha, which rendered as a white box/halo on every
  non-white surface (window/taskbar icon, sidebar, launchers)
- regenerate both as RGBA: alpha is the ink coverage, RGB forced to 0 so
  no white matte can leak through semi-transparent edge pixels
- styles.css: drop the white tile background/border-radius and cover-fit
  from .sidebar-logo; the artwork now composites directly (contain-fit)
- originals preserved under deferred/original-icons/

frontend/src/styles.css

@@ -808,22 +808,22 @@ a {
 .sidebar-logo {
   width: 38px;
   height: 38px;
-  border-radius: 11px;
   display: grid;
   place-items: center;
-  background: #fff;
+  /* The logo PNG now carries a real alpha channel: no tile background,
+     border, or mask — the artwork composites directly on the sidebar. */
   overflow: hidden;
 }
 
 .sidebar-logo img {
   width: 100%;
   height: 100%;
-  object-fit: cover;
+  object-fit: contain;
   display: block;
 }
 
-/* The artwork is black-on-white; flip it in dark themes so it stays black
-   bird on dark tile instead of a glaring white square. */
+/* The artwork is black ink; flip it in dark themes so it stays visible on
+   dark sidebars. */
 html[data-theme='dark'] .sidebar-logo img,
 html[data-theme='neon'] .sidebar-logo img,
 html[data-theme='glass'] .sidebar-logo img {