Expose raw CLI startup details in desktop error cards so Windows failures are diagnosable

The chat UI translated CLI startup failures to a generic localized string and
hid the server-provided detail entirely. That made remote Windows debugging
guesswork even when the backend had already captured the real stderr payload.

This change keeps the localized summary but renders the raw startup detail
underneath when it differs, and adds a regression test covering the Git Bash
missing case.

Constraint: Windows Server repro is happening remotely, so the immediate value is better diagnostics rather than guessing a root cause locally
Rejected: Leave the generic translation only | obscures actionable stderr and slows every remote repro cycle
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: For translated startup errors, preserve backend detail somewhere visible unless it duplicates the summary
Tested: cd desktop && bunx vitest run src/components/chat/MessageList.test.tsx
Tested: cd desktop && bun run lint
Not-tested: Full desktop interaction flow on Windows Server 2022
Related: GitHub issue #62

desktop/src/components/chat/MessageList.test.tsx

@@ -281,4 +281,32 @@ describe('MessageList nested tool calls', () => {
       '先看 CLI 和服务端入口。\n再看 desktop 前后端边界。'
     )
   })
+
+  it('shows raw startup details under translated CLI startup errors', () => {
+    useChatStore.setState({
+      sessions: {
+        [ACTIVE_TAB]: makeSessionState({
+          messages: [
+            {
+              id: 'error-1',
+              type: 'error',
+              code: 'CLI_START_FAILED',
+              message:
+                'CLI exited during startup (code 1): Claude Code on Windows requires git-bash (https://git-scm.com/downloads/win).',
+              timestamp: 1,
+            },
+          ],
+        }),
+      },
+    })
+
+    render(<MessageList />)
+
+    expect(screen.getByText('Failed to start CLI process.')).toBeTruthy()
+    expect(
+      screen.getByText(
+        'CLI exited during startup (code 1): Claude Code on Windows requires git-bash (https://git-scm.com/downloads/win).',
+      ),
+    ).toBeTruthy()
+  })
 })

desktop/src/components/chat/MessageList.tsx

@@ -222,9 +222,18 @@ export const MessageBlock = memo(function MessageBlock({
       const errorKey = message.code ? `error.${message.code}` as TranslationKey : null
       const errorText = errorKey ? t(errorKey) : null
       const displayMessage = (errorText && errorText !== errorKey) ? errorText : message.message
+      const showRawDetail =
+        Boolean(message.message) &&
+        message.message.trim() !== '' &&
+        message.message !== displayMessage
       return (
         <div className="mb-3 px-4 py-2.5 rounded-lg bg-red-50 border border-red-200 text-sm text-[var(--color-error)]">
           <strong>Error:</strong> {displayMessage}
+          {showRawDetail && (
+            <div className="mt-1 whitespace-pre-wrap text-xs text-red-700/85">
+              {message.message}
+            </div>
+          )}
         </div>
       )
     }