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/web/src/base.css

@@ -1,9 +1,10 @@
 /* Shell-owned global base: full-height mount plus the theme token sheets.
- * The three ui-theme sheets are the sole token source (--dsw-*); the shell
+ * The four ui-theme sheets are the sole token source (--dsw-*); the shell
  * links them here so tokens exist before any plugin CSS lands. */
 @import '@deepseek-ai/dsh-client-ui-theme/styles/base.css';
 @import '@deepseek-ai/dsh-client-ui-theme/styles/design-platform.css';
 @import '@deepseek-ai/dsh-client-ui-theme/styles/gradient-shadow-text.css';
+@import '@deepseek-ai/dsh-client-ui-theme/styles/shiki.css';
 
 html,
 body,