refactor: strip useUnistyles in favor of withUnistyles + StyleSheet

Replaces broad useUnistyles() subscriptions in composer, workspace, and
related components with withUnistyles wrappers and StyleSheet.create so
re-renders stay scoped to the tokens actually consumed. Lifts the design
scales (spacing, iconSize, fontSize, etc.) to top-level exports in
theme.ts so component literals share a single source of truth with the
unistyles theme.

packages/app/src/components/message-input.test.tsx

@@ -47,7 +47,7 @@ vi.mock("react-native-unistyles", () => ({
   StyleSheet: {
     create: (factory: unknown) => (typeof factory === "function" ? factory(theme) : factory),
   },
-  useUnistyles: () => ({ theme }),
+  withUnistyles: <T,>(component: T) => component,
 }));
 
 vi.mock("@/constants/platform", () => ({