feat(web): shiki syntax highlighting for code surfaces

One highlighter for the client: a synchronous fine-grained shiki core
(JS regex engine, no WASM) in ui-primitives with an explicit grammar
allowlist (typescript, shellscript, json — aliases resolve, unknown
languages take a geometry-identical plain arm). The shared CodeBlock
component owns both arms; markdown fences, the run_code expanded
program body (typescript), and the details panel Input (json) all
route through it. Token colors live in a new ui-theme shiki.css sheet
as --shiki-* custom properties (light/dark blocks), wired through the
shell's base.css chain — tokens-only styling holds; shiki's generated
span tree is the sanctioned innerHTML path (static output, no user
HTML). jsdom specs pin token spans, aliases, both fallbacks, and the
fence route; the built-bundle snapshot asserts the highlighted program
under the code row.

packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx

@@ -152,7 +152,7 @@ describe('run_code sub-calls through the real chat machinery', () => {
     expect(view.getByText('Tool call')).toBeTruthy()
   })
 
-  it('expanding the code row reveals the program body verbatim', async () => {
+  it('expanding the code row reveals the program body verbatim (shiki-tokenized)', async () => {
     const parent = 'call-64'
     const b = await bench(snapshotWith([codeResult(10, parent)], new Map()))
     const view = mountApp(b.slots)
@@ -160,7 +160,12 @@ describe('run_code sub-calls through the real chat machinery', () => {
     const toggle = view.container.querySelector('[data-variant="code"] button[aria-expanded]')
     expect(toggle).not.toBeNull()
     fireEvent.click(toggle!)
-    expect(view.getByText(/const listing = await tools\.bash/)).toBeTruthy()
+    // Shiki splits the program into token spans inside one <pre class="shiki">:
+    // assert the whole text and the highlighted tree rather than one node.
+    const pre = view.container.querySelector('pre.shiki')
+    expect(pre).not.toBeNull()
+    expect(pre!.textContent).toContain('const listing = await tools.bash')
+    expect(pre!.querySelectorAll('span[style]').length).toBeGreaterThan(3)
   })
 
   it('an isError sub-call renders the error state dot exactly like a failed native row', async () => {