fix(desktop): avoid stack overflow rendering huge fenced blocks

`normalizeFenceBlocks`/`pushProseFence` appended block bodies with
`out.push(...lines)`, which spreads every line as a separate call
argument. A single message carrying a large fenced block (a logged
minified bundle, base64 blob, or big tool dump — common in long
sessions) overflows V8's argument-count limit and throws
`RangeError: Maximum call stack size exceeded`, breaking the transcript
render. Compression doesn't save us: it gates on tokens vs. window, not
a single message's line count, and the protected recent tail renders
verbatim regardless.

Append iteratively via a small `extend()` helper. Behavior is identical
for normal-sized blocks.

apps/desktop/src/components/assistant-ui/markdown-text.test.ts

@@ -201,4 +201,13 @@ describe('preprocessMarkdown', () => {
 
     expect(output).toContain('<https://example.com/a_b/c~d/page>')
   })
+
+  it('handles a fenced block larger than V8 spread-argument limit', () => {
+    // A single huge code block (e.g. a logged minified bundle) used to throw
+    // `RangeError: Maximum call stack size exceeded` via `out.push(...lines)`.
+    const body = Array.from({ length: 200_000 }, (_, i) => `line ${i}`).join('\n')
+    const input = `\`\`\`js\n${body}\n\`\`\``
+
+    expect(() => preprocessMarkdown(input)).not.toThrow()
+  })
 })

apps/desktop/src/lib/markdown-preprocess.ts

@@ -151,12 +151,22 @@ function normalizeVisibleProse(text: string): string {
     .join('')
 }
 
+// `out.push(...lines)` spreads every element as a separate call argument, so a
+// single fenced block with tens of thousands of lines (a logged minified
+// bundle, base64 blob, huge tool dump) overflows V8's argument-count limit and
+// throws `RangeError: Maximum call stack size exceeded`. Append iteratively.
+function extend(out: string[], lines: string[]) {
+  for (const line of lines) {
+    out.push(line)
+  }
+}
+
 function pushProseFence(out: string[], indent: string, info: string, lines: string[]) {
   if (info) {
     out.push(`${indent}${info}`.trimEnd())
   }
 
-  out.push(...lines)
+  extend(out, lines)
 }
 
 function findClosingFence(lines: string[], start: number, marker: string): number {
@@ -241,7 +251,7 @@ function normalizeFenceBlocks(text: string): string {
     }
 
     if (closeIndex !== -1 && isUrlOnlyBlock(bodyLines)) {
-      out.push(...bodyLines)
+      extend(out, bodyLines)
       index = closeIndex + 1
 
       continue
@@ -264,10 +274,10 @@ function normalizeFenceBlocks(text: string): string {
         // any literal `$$` characters in the body don't collide with
         // an outer math wrapper. No close emitted yet — streaming.
         out.push(`${indent}${marker}math`)
-        out.push(...bodyLines)
+        extend(out, bodyLines)
       } else {
         out.push(`${indent}${marker}${language}`)
-        out.push(...bodyLines)
+        extend(out, bodyLines)
       }
 
       break
@@ -288,7 +298,7 @@ function normalizeFenceBlocks(text: string): string {
       // colliding with our wrapper. Without this rewrite the block
       // would render as a syntax-highlighted "latex" code listing.
       out.push(`${indent}${marker}math`)
-      out.push(...bodyLines)
+      extend(out, bodyLines)
       out.push(`${indent}${marker}`)
       index = closeIndex + 1
 
@@ -296,7 +306,7 @@ function normalizeFenceBlocks(text: string): string {
     }
 
     out.push(`${indent}${marker}${language}`)
-    out.push(...bodyLines)
+    extend(out, bodyLines)
     out.push(`${indent}${marker}`)
     index = closeIndex + 1
   }