fix(studio): single-frame export uses wrong content, ignores duration setting

1. Wrong content: exportMp4 always rendered the template's original
   source HTML, ignoring the agent-generated preview saved to
   lastPreviewHtmlPath. Now uses lastPreviewHtmlPath when present.

2. Duration ignored: exportMp4 hardcoded `duration: 'auto'`, causing
   the adapter to fall back to the animation's natural length (~4s)
   instead of the user's setting. durationTargetSec was defined in
   UserPreferences but never written or read.
   - studio-server now persists collected.duration to
     preferences.durationTargetSec at generate time (covers both
     single-frame and multi-frame paths).
   - exportMp4 reads durationTargetSec and passes it as
     durationMode: 'explicit' so the adapter honors it as a hard cap.

3. UI showed render time not video duration: the "MP4 已导出 · Xs"
   badge was displaying elapsed_ms (wall-clock render time) instead
   of the actual video length. export_done SSE event now carries
   duration_sec; the studio UI uses that instead.

Author: walliai666

packages/cli/src/studio-server.ts

@@ -398,7 +398,11 @@ export async function startStudioServer(ctx: CliContext, port: number): Promise<
           process.stderr.write(
             `[studio:export] proj=${projectId} done in ${ms}ms → ${outputPath}\n`,
           );
-          sse({ type: 'export_done', output_path: outputPath, project, elapsed_ms: ms });
+          // Compute the actual video duration to show in the UI (not the render wall-clock time).
+          const videoDurationSec = (project.frames && project.frames.length > 0)
+            ? project.frames.reduce((s, f) => s + (f.durationSec || 0), 0)
+            : (project.preferences.durationTargetSec ?? null);
+          sse({ type: 'export_done', output_path: outputPath, project, elapsed_ms: ms, duration_sec: videoDurationSec });
         } catch (err) {
           const msg = err instanceof Error ? err.message : String(err);
           process.stderr.write(`[studio:export] proj=${projectId} failed: ${msg}\n`);
@@ -958,6 +962,16 @@ export async function startStudioServer(ctx: CliContext, port: number): Promise<
         let textChunks = 0;
         let summaryLine = '';
 
+        // Persist user duration preference at generate time so exportMp4 can
+        // honor it regardless of single-frame vs multi-frame path.
+        if (phaseInfo.phase === 'generate' && phaseInfo.inputs.collected) {
+          const parsedDur = Number(phaseInfo.inputs.collected.duration ?? '') || 0;
+          if (parsedDur > 0) {
+            project.preferences = { ...project.preferences, durationTargetSec: parsedDur };
+            await ctx.projects.save(project);
+          }
+        }
+
         // ---- generate-phase: multi-frame path runs split (graph + per-frame) ----
         // Empirically claude --print returns 1 byte ~50% of the time when asked
         // to emit a graph and 4-6 full HTML pages in a single response. Each

packages/core/src/project.ts

@@ -443,15 +443,24 @@ export class ProjectOrchestrator {
     const tmpl = this.deps.templates.get(project.templateId);
     const adapter = this.deps.engines.get(tmpl.engine);
 
+    // Use the agent-generated HTML if available; fall back to the template source.
+    const singleFrameTemplateRef = project.lastPreviewHtmlPath
+      ? { id: tmpl.id, engine: tmpl.engine, sourcePath: project.lastPreviewHtmlPath }
+      : templateRefFromMeta(tmpl);
+    // Honor the user-set duration; fall back to 'auto' so the adapter probes animation length.
+    const singleFrameDuration = project.preferences.durationTargetSec ?? 'auto';
+    const singleFrameDurationMode = typeof singleFrameDuration === 'number' ? 'explicit' as const : undefined;
+
     await adapter.render(
       {
-        template: templateRefFromMeta(tmpl),
+        template: singleFrameTemplateRef,
         variables: project.variables,
         config: {
           format: 'mp4',
           resolution: project.preferences.resolution ?? { width: 1920, height: 1080 },
           fps: project.preferences.fps ?? 60,
-          duration: 'auto',
+          duration: singleFrameDuration,
+          ...(singleFrameDurationMode && { durationMode: singleFrameDurationMode }),
           outputPath,
         },
       },

packages/project-studio/public/app.js

@@ -192,7 +192,7 @@ async function startExportStream() {
           state.exporting = false;
           state.exportProgress = null;
           if (ev.project) state.selected = ev.project;
-          const seconds = ev.elapsed_ms ? `${(ev.elapsed_ms / 1000).toFixed(1)}s` : '';
+          const seconds = ev.duration_sec != null ? `${Number(ev.duration_sec).toFixed(1)}s` : '';
           state.messages.push({
             role: 'preview-event',
             content: seconds ? t('export.done_seconds', { seconds }) : t('export.done_no_seconds'),