fix: center minichat widget and fix weather tool success status

- KnowledgeView minichat: add margin-inline: auto so the widget centers
  within the content area when max-width is reached on wide screens
- weather get_weather tool: return success: true on both the arbitrary
  location path and the cached locations path so ToolCallCard shows
  the correct success state instead of always flagging as error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/KnowledgeView.vue

@@ -1043,6 +1043,7 @@ onUnmounted(() => {
   left: var(--sidebar-width); /* align with content area past filter panel */
   right: 0;
   max-width: var(--page-max-width);
+  margin-inline: auto;
   z-index: 20;
   display: flex;
   flex-direction: column;