feat(app): syntax highlighting and copy button for chat code blocks (#1062)

Adds Lezer-based highlighting to fenced code blocks in assistant messages,
with a hover/tap copy button. Colors move into the theme as
`theme.colors.syntax`, so the chat blocks, file preview, and diff viewer
share one StyleSheet path — token color updates flow through the Unistyles
native ShadowRegistry with no React re-renders on the chat hot path.

packages/app/src/styles/theme.ts

@@ -1,3 +1,5 @@
+import { darkHighlightColors, lightHighlightColors } from "@getpaseo/highlight";
+
 export const baseColors = {
   // Base colors
   white: "#ffffff",
@@ -518,6 +520,7 @@ function buildDarkTheme(semanticColors: ReturnType<typeof buildDarkSemanticColor
     colors: {
       ...semanticColors,
       palette: baseColors,
+      syntax: darkHighlightColors,
     },
     shadow: darkShadow,
     ...commonTheme,
@@ -535,6 +538,7 @@ export const lightTheme = {
   colors: {
     ...lightSemanticColors,
     palette: baseColors,
+    syntax: lightHighlightColors,
   },
   shadow: {
     sm: {