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-primitives/tests/markdown.spec.tsx

@@ -57,6 +57,8 @@ describe('MarkdownText', () => {
     expect(container.querySelector('table')?.textContent).toContain('alphabeta')
     expect(container.querySelector('hr')).not.toBeNull()
     expect(container.querySelector('pre code')?.textContent).toContain('const answer = 42')
+    // The ts fence routed through the shared CodeBlock: shiki token spans present.
+    expect(container.querySelector('pre.shiki')).not.toBeNull()
     expect(container.querySelector('br')).not.toBeNull()
     expect(screen.getByRole('link', { name: 'safe' }).getAttribute('target')).toBe('_blank')
     expect(screen.getByRole('link', { name: 'https://deepseek.com' })).toBeTruthy()