feat(ui): add system/light/dark theme toggle

Add a 3-state theme toggle in folder selection and instance tabs, and update tokens/styles so light mode has readable contrast. Sync MUI surfaces and Shiki highlighting to CSS variables to prevent stale colors when switching themes.

packages/ui/src/components/message-part.tsx

@@ -103,15 +103,15 @@ interface MessagePartProps {
       <Match when={partType() === "text"}>
         <Show when={!shouldHideTextPart() && partHasRenderableText(props.part)}>
           <div class={textContainerClass()}>
-            <Show
-               when={isAssistantMessage()}
-               fallback={<span>{plainTextContent()}</span>}
-             >
-              <Markdown
-                part={createTextPartForMarkdown()}
-                instanceId={props.instanceId}
-                sessionId={props.sessionId}
-                isDark={isDark()}
+               <Show
+                when={isAssistantMessage()}
+                fallback={<span class="text-primary">{plainTextContent()}</span>}
+              >
+                <Markdown
+                  part={createTextPartForMarkdown()}
+                  instanceId={props.instanceId}
+                  sessionId={props.sessionId}
+                  isDark={isDark()}
                 size={isAssistantMessage() ? "tight" : "base"}
                 onRendered={props.onRendered}
               />