fix(vikunja): use the real Vikunja logo mark

Replaces my drawn placeholder with the actual logo path supplied by the
maintainer.

Kept as .svg rather than .ico: webuiSyncAppIcon only looks for
<app>.svg then <app>.png, so an .ico would never be copied into the
frontend and the app would silently fall back to the default icon. SVG
is also the right format here — it is a single vector path that scales
to any tile size, where an .ico would bake in fixed raster sizes.

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

containers/vikunja/vikunja.svg

@@ -1,9 +1,3 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 48 48" width="48" height="48" role="img" aria-label="Vikunja">
-  <!-- Placeholder mark: a checklist on a board. Drawn rather than copied so no
-       upstream trademark is vendored into the repo. -->
-  <rect x="7" y="6" width="34" height="36" rx="4" fill="none" stroke="#1973ff" stroke-width="2.5"/>
-  <path d="M14 16.5l3 3 6-6" fill="none" stroke="#1973ff" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M14 27.5l3 3 6-6" fill="none" stroke="#1973ff" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"/>
-  <line x1="27" y1="17" x2="35" y2="17" stroke="#1973ff" stroke-width="2.5" stroke-linecap="round"/>
-  <line x1="27" y1="28" x2="35" y2="28" stroke="#1973ff" stroke-width="2.5" stroke-linecap="round"/>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256" width="256" height="256" role="img" aria-label="Vikunja">
+  <path fill="#196aff" d="M226.9 46.8A126 126 0 0 0 128 0a127.2 127.2 0 0 0-99 46.8A127 127 0 0 0 0 128c0 34.2 13.3 66.3 37.5 90.5A127.2 127.2 0 0 0 128 256c34.2 0 66.3-13.3 90.5-37.5A127.2 127.2 0 0 0 256 128a130 130 0 0 0-29.1-81.2"/>
 </svg>