feat(FN-4611): add themed quick chat session dropdown

- Replace the quick chat header mode toggle with a session picker dropdown styled from dashboard theme tokens
- Surface chat room entries alongside sessions and keep active room/session switching in the same menu
- Add QuickChatFAB and CSS regression coverage for the new dropdown behavior and token-based styling
- Add a changeset note for the quick chat session picker update

Fusion-Task-Id: FN-4611
Fusion-Task-Id: FN-4634
Fusion-Task-Lineage: 87cc75e7-ba50-4cb5-bb9f-6d86f49268e0

packages/dashboard/app/__tests__/quick-chat-session-dropdown.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+import { loadAllAppCss } from "../test/cssFixture";
+
+describe("QuickChat session dropdown CSS", () => {
+  it("defines themed dropdown selectors using tokens", async () => {
+    const css = await loadAllAppCss();
+
+    const triggerBlock = css.match(/\.quick-chat-session-trigger\s*\{[^}]*\}/)?.[0] ?? "";
+    const dropdownBlock = css.match(/\.quick-chat-session-dropdown\s*\{[^}]*\}/)?.[0] ?? "";
+
+    expect(triggerBlock).toContain(".quick-chat-session-trigger");
+    expect(dropdownBlock).toContain(".quick-chat-session-dropdown");
+
+    const combined = `${triggerBlock}\n${dropdownBlock}`;
+    expect(combined).not.toMatch(/#[0-9a-fA-F]{3,8}\b/);
+    expect(combined).not.toMatch(/rgba?\(/);
+    expect(combined).not.toMatch(/(?<![\w-])(?:[1-9]\d*|\d+\.\d+)px\b/);
+  });
+});