fix(llm): stop leaking instruction prompts instead of processed text

Running a custom instruction (translate, summarise, rewrite, explain code,
free prompt) inserted the instruction's own wording instead of the result.
Two faults stacked:

The instruction was passed as the text to process, leaving the system-prompt
argument empty. `BASE_SYSTEM_PROMPTS` has no `custom` key, so resolution fell
back to `refine` without saying so, and the model dutifully polished the
instruction it had been handed. The transcript never reached it.

And only `{{text}}` was substituted, which none of the five built-in
instructions use — they carry `{{targetLanguage}}`, `{{userPrompt}}`, or no
placeholder at all. The substitution was a no-op from the day it was written:
the presets landed ten hours before the code that expected them.

- Instruction prompts now go to the system-prompt argument and the transcript
  to the text argument. Instructions that spell out `{{text}}` keep their old
  meaning, so hand-written ones still work.
- `renderInstructionPrompt` resolves `{{text}}`, `{{userPrompt}}` and
  `{{targetLanguage}}` in one place, and warns by name when a placeholder is
  left standing rather than letting it reach the model.
- `resolveSystemPrompt` no longer drops silently to `refine` for `custom`.
- Voice shortcuts no longer die at the `defaultLLMAction === 'none'` gate; an
  explicitly named instruction outranks the default. Without one, `none` still
  passes the transcript through untouched.
- `translate` receives its target language instead of relying on a default two
  call frames away. It is still always English — `AppConfig` has no key for it,
  and neither `language` (UI locale) nor `sttLanguage` (source language) can
  stand in. Choosing a target language needs a setting and is not in this fix.
- Chains ran instructions with placeholders intact; they share the same
  resolution now.
- The command screen's pipeline bench called `llm.generate`, which preload does
  not expose, so every run threw and the catch showed the input back as if it
  had succeeded. It uses `llm.process` now, over the same path production
  takes, and a failure reads as a failure.

Present since the feature shipped: the custom-instruction path has never
worked. Plain actions (refine, summarise, grammar, expand) were unaffected and
are now covered by tests so they stay that way.

apps/desktop/src/renderer/pages/CommandsPage.tsx

@@ -26,6 +26,7 @@ import {
   Mic,
   Cpu,
   ClipboardCopy,
+  TriangleAlert,
 } from 'lucide-react'
 import {
   MetalCard,
@@ -68,6 +69,7 @@ export function CommandsPage(): React.ReactElement {
   // Interactive Pipeline Test State
   const [testInput, setTestInput] = useState('')
   const [testResult, setTestResult] = useState<string | null>(null)
+  const [testError, setTestError] = useState<string | null>(null)
   const [testing, setTesting] = useState(false)
 
   const loadData = useCallback(async () => {
@@ -175,23 +177,28 @@ export function CommandsPage(): React.ReactElement {
     }
   }
 
+  // 이 벤치는 실제 받아쓰기와 같은 경로(llm.process → processText)를 탄다.
+  // 지시문 원문을 customPrompt로 넘기면 메인 프로세스가 플레이스홀더 치환과
+  // 시스템 프롬프트 배치를 처리한다 — 치환 규칙을 렌더러에 복제하지 않는다.
   const handleRunPipelineTest = async () => {
     if (!testInput.trim()) return
     setTesting(true)
     setTestResult(null)
+    setTestError(null)
     try {
       const activeInst = instructions.find((i) => i.id === activeId)
-      const prompt = activeInst?.prompt || '다음 문장을 자연스럽고 깔끔하게 다듬어주세요:'
-      const res = await window.electronAPI.llm.generate({
-        prompt: `${prompt}\n\n${testInput.trim()}`,
-      })
+      const res = await window.electronAPI.llm.process(
+        activeInst
+          ? { text: testInput.trim(), action: 'custom', customPrompt: activeInst.prompt }
+          : { text: testInput.trim(), action: 'refine' },
+      )
       if (res.success) {
-        setTestResult(res.data.text)
+        setTestResult(res.data.processedText)
       } else {
-        setTestResult(testInput.trim())
+        setTestError(res.error.message || t('popup.error.default'))
       }
-    } catch {
-      setTestResult(testInput.trim())
+    } catch (error) {
+      setTestError(error instanceof Error ? error.message : t('popup.error.default'))
     } finally {
       setTesting(false)
     }
@@ -543,6 +550,25 @@ export function CommandsPage(): React.ReactElement {
                 </Typography>
               </Box>
             )}
+
+            {testError && (
+              <Box
+                sx={{
+                  p: 2,
+                  bgcolor: d3roPalette.status.dangerBg,
+                  borderRadius: d3roRadius.inner,
+                  border: `1px solid ${d3roPalette.status.danger}`,
+                  display: 'flex',
+                  alignItems: 'flex-start',
+                  gap: 1.5,
+                }}
+              >
+                <TriangleAlert size={16} style={{ color: d3roPalette.status.danger, marginTop: 2, flexShrink: 0 }} />
+                <Typography sx={{ fontFamily: d3roFontSans, fontSize: '13.5px', color: d3roPalette.status.danger, lineHeight: 1.6 }}>
+                  {testError}
+                </Typography>
+              </Box>
+            )}
           </Box>
         </MetalCard>
       </Box>