fix(timeline): 修时间轴无法滚动(flex min-width:auto 坑) + 加鼠标拖拽平移

- 根因：.tl-tracks 作为 flex 子项默认 min-width:auto，被 ~3000px 内容撑大不收缩，
  内部 overflow-x 永不触发→滚轮/滚动条全失效。修复=加 min-width:0
- 新增鼠标按住拖拽平移时间轴(超阈值不算点击，避免误选)；滚轮横向滚动；cursor:grab

web/static/timeline.js

@@ -315,6 +315,12 @@
     PX = fitMode ? Math.max(6, ((host.clientWidth || 760) - 60) / Math.max(model.total, 0.1)) : PXMAX;
     // 鼠标滚轮 → 横向滚动（横向溢出时）
     host.onwheel = e => { if (host.scrollWidth > host.clientWidth + 1) { host.scrollLeft += (e.deltaY || 0) + (e.deltaX || 0); e.preventDefault(); } };
+    // 鼠标按住拖拽 → 平移时间轴（拖动超过阈值则不算点击，避免误选）
+    let dragging = false, dragX = 0, dragScroll = 0, moved = false;
+    host.onmousedown = e => { if (e.button !== 0) return; dragging = true; moved = false; dragX = e.clientX; dragScroll = host.scrollLeft; };
+    host.onmousemove = e => { if (!dragging) return; const dx = e.clientX - dragX; if (Math.abs(dx) > 3) { moved = true; host.style.cursor = "grabbing"; } host.scrollLeft = dragScroll - dx; e.preventDefault(); };
+    const endDrag = () => { dragging = false; host.style.cursor = ""; };
+    host.onmouseup = endDrag; host.onmouseleave = endDrag;
     const W = model.total * PX;
     const ruler = document.createElement("div"); ruler.className = "tl-ruler"; ruler.style.width = W + "px";
     for (let s = 0; s <= Math.ceil(model.total); s++) {
@@ -335,7 +341,7 @@
         el.textContent = c.label;
         el.dataset.start = c.start; el.dataset.end = c.start + c.dur;
         if (c.nodeId && S.nodes[c.nodeId]) el.classList.add("startable");
-        el.onclick = e => { e.stopPropagation(); selectClip(c, el); };       // 单击=选中(不跳时间，避免视图乱滚)
+        el.onclick = e => { e.stopPropagation(); if (moved) return; selectClip(c, el); };   // 拖动后不算选中
         el.ondblclick = e => { e.stopPropagation(); e.preventDefault(); if (c.nodeId && S.nodes[c.nodeId]) startFrom(c.nodeId, true); };
         lane.appendChild(el);
       });
@@ -343,6 +349,7 @@
     });
     const ph = document.createElement("div"); ph.className = "tl-playhead"; host.appendChild(ph); els.playhead = ph;
     host.onclick = e => {
+      if (moved) return;   // 刚才是拖拽，不当作跳转
       const rect = host.getBoundingClientRect();
       seek(Math.max(0, Math.min(model.total, (e.clientX - rect.left + host.scrollLeft) / PX)));
     };