fix(frontend): fill brand mark box with favicon tile

The sidebar and login logo boxes kept a #1c2a3d background around a
smaller favicon image, leaving an inconsistent ring in dark mode. The
image now fills the box and the divergent box background is removed, so
the favicon's own black tile is the visual mark.

Co-authored-by: Codex <noreply@openai.com>

frontend/src/layout/components/AppSidebar.vue

@@ -339,12 +339,12 @@ onBeforeUnmount(() => {
       <BaseMotionIcon
         aria-hidden="true"
         variant="brand"
-        class="inline-flex size-11 items-center justify-center relative -top-0.5 rounded-(--cp-icon-button-radius) bg-(--cp-brand-mark-bg)"
+        class="inline-flex size-11 items-center justify-center relative -top-0.5 rounded-(--cp-icon-button-radius)"
       >
         <img
           src="/favicon.svg"
           alt=""
-          class="block size-9 select-none"
+          class="block size-11 select-none"
           draggable="false"
         >
       </BaseMotionIcon>

frontend/src/views/login/components/LoginPanel.vue

@@ -61,7 +61,7 @@ function togglePasswordVisible(): void {
           <img
             src="/favicon.svg"
             alt=""
-            class="block size-8 select-none"
+            class="block size-[38px] select-none"
             draggable="false"
           >
         </BaseMotionIcon>
@@ -232,7 +232,6 @@ function togglePasswordVisible(): void {
   align-items: center;
   justify-content: center;
   border-radius: 8px;
-  background: var(--login-logo-bg);
   color: var(--login-logo-text);
   font-family: var(--font-mono);
   font-size: 12px;