feat(ui): syntax-highlight the read-only source previews (#1284)

The read-only previews beside Problems, Show Usages and TODO results are syntax-coloured from one whole-file parse (capped for very large files), so a line inside a block comment or a multi-line string is coloured as such; the pane renders plain first and recolours when the parse lands. SourcePreview takes the colouring as a colorize prop (the editor's previewColors), keeping src/ui free of editor imports, and the tok-* colours live once in global.css instead of a copy per view, with the theme's comment style.

Co-authored-by: qtvbwfn <[email protected]>

src/features/agents/Session.module.css

@@ -307,44 +307,6 @@
   text-decoration: underline;
 }
 
-/* Lezer classHighlighter tokens in fenced code (same colours as the Markdown preview). */
-.markdown :global(.tok-keyword),
-.markdown :global(.tok-modifier),
-.markdown :global(.tok-controlKeyword),
-.markdown :global(.tok-operatorKeyword) {
-  color: var(--syn-keyword);
-}
-.markdown :global(.tok-string),
-.markdown :global(.tok-string2) {
-  color: var(--syn-string);
-}
-.markdown :global(.tok-number),
-.markdown :global(.tok-bool),
-.markdown :global(.tok-atom) {
-  color: var(--syn-number);
-}
-.markdown :global(.tok-typeName),
-.markdown :global(.tok-className),
-.markdown :global(.tok-namespace) {
-  color: var(--syn-type);
-}
-.markdown :global(.tok-comment) {
-  color: var(--syn-comment);
-}
-.markdown :global(.tok-meta),
-.markdown :global(.tok-macroName) {
-  color: var(--syn-annotation);
-}
-.markdown :global(.tok-propertyName) {
-  color: var(--syn-property);
-}
-.markdown :global(.tok-variableName.tok-definition) {
-  color: var(--syn-function);
-}
-.markdown :global(.tok-punctuation),
-.markdown :global(.tok-operator) {
-  color: var(--syn-punct);
-}
 .markdown :global(ul),
 .markdown :global(ol) {
   margin: 0 0 8px;