perf(markdown): move code highlighting off the main thread into a Shiki worker

Tokenize closed code blocks in a dedicated Shiki Web Worker instead of calling
the shared highlighter synchronously on the UI thread. This removes the
one-shot main-thread highlight stall when a code fence closes on a large block.

Streaming behavior is unchanged: the open (streaming) fence still renders as
plain text and is highlighted once on close. On any worker failure the block
keeps its escaped plain code — highlighting never falls back onto the main
thread.

- Add markdownShikiThemeDefinition (dependency-free CSS-variable theme) so the
  worker can use the theme without pulling in @pierre/diffs / React.
- Add markdown-worker-protocol, markdown-shiki.worker, and the main-thread
  markdown-worker client.
- Route highlightCodeBlocks through the worker; keep the size/VSCode line guard
  and mermaid skip on the main thread.
- Add shiki as a direct dependency (was transitive via @pierre/diffs).

bun.lock

@@ -99,7 +99,7 @@
     },
     "packages/electron": {
       "name": "@openchamber/electron",
-      "version": "1.12.4",
+      "version": "1.13.0",
       "dependencies": {
         "@openchamber/web": "workspace:*",
         "electron-context-menu": "^4.1.2",
@@ -114,7 +114,7 @@
     },
     "packages/ui": {
       "name": "@openchamber/ui",
-      "version": "1.12.4",
+      "version": "1.13.0",
       "dependencies": {
         "@base-ui/react": "^1.4.0",
         "@codemirror/autocomplete": "^6.20.0",
@@ -178,6 +178,7 @@
         "rehype-katex": "^7.0.1",
         "remark-math": "^6.0.0",
         "remend": "^1.2.1",
+        "shiki": "^3.23.0",
         "simple-git": "^3.28.0",
         "sonner": "^2.0.7",
         "strip-json-comments": "^5.0.3",
@@ -216,7 +217,7 @@
     },
     "packages/vscode": {
       "name": "openchamber",
-      "version": "1.12.4",
+      "version": "1.13.0",
       "dependencies": {
         "@openchamber/ui": "workspace:*",
         "@opencode-ai/sdk": "^1.17.7",
@@ -239,7 +240,7 @@
     },
     "packages/web": {
       "name": "@openchamber/web",
-      "version": "1.12.4",
+      "version": "1.13.0",
       "bin": {
         "openchamber": "./bin/cli.js",
       },