fix(stalwart): use the official logo instead of my placeholder

Replaces the drawn shield-and-envelope stand-in with the real mark from
stalw.art (/favicon.svg), in their #DB2D54.

Padded from the source's 159.95x139.07 to a square 159.95 viewBox with
the art vertically centred, matching every other catalogue icon — all of
which are square, so a non-square box would letterbox in the app tiles.

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

containers/stalwart/stalwart.svg

@@ -1,11 +1,8 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 48 48" width="48" height="48" role="img" aria-label="Stalwart Mail Server">
-  <!-- Placeholder mark: a shield (stalwart) around an envelope (mail), drawn
-       rather than copied so no upstream trademark is vendored into the repo.
-       Swap for the official logo if the project's brand terms allow it. -->
-  <path d="M24 3 40 8v14c0 10-6.6 18.6-16 23C14.6 40.6 8 32 8 22V8z"
-        fill="none" stroke="#2f6fed" stroke-width="2.5" stroke-linejoin="round"/>
-  <rect x="14" y="17" width="20" height="14" rx="2"
-        fill="none" stroke="#2f6fed" stroke-width="2.5"/>
-  <path d="M14.5 18.5 24 26l9.5-7.5"
-        fill="none" stroke="#2f6fed" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"/>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 -10.44 159.95 159.95" width="159.95" height="159.95" fill="#DB2D54" role="img" aria-label="Stalwart">
+  <g transform="translate(-46.07, -56.57)">
+    <path d="M130.1,56.6H122l-75.9,43.8v14l75.9,43.8h8.1l75.9-43.8v-14L130.1,56.6z M185.8,107.4h-59.7V72.9L185.8,107.4z M126.1,141.9l-59.7-34.5L112.1,81v26.4v7v0.8l10.7,6.2h38.8L126.1,141.9z"/>
+    <polygon points="46.1,133.1 46.1,151.7 122,195.6 130.1,195.6 206,151.7 206,133.1 126.1,179.2"/>
+    <polygon points="194.8,195.6 206,189.1 206,170.4 162.5,195.6"/>
+    <polygon points="46.1,189.1 57.3,195.6 89.7,195.6 46.1,170.4"/>
+  </g>
 </svg>