fix(ui): left-click-drag on empty canvas pans the view

Canvas zoom shipped pan as middle-drag / Space+drag, which left m unable
to reach a freshly-created frame outside the default viewport — the
only escape was middle-button or holding Space, neither of which is
discoverable.

Empty-canvas left-pointerdown now starts an ambiguous gesture: if the
cursor moves past a 3px screen-space threshold it promotes to a pan
(Excalidraw / Figma standard); below the threshold it falls back to
the historic "click empties the selection" UX so plain clicks still
deselect. Pointerdown on a device, frame, IO marker, port, or cable
keeps routing to its own handler. Middle-drag and Space+drag pan
unchanged.

web/static/main.js

@@ -271,6 +271,52 @@ function startPan(e) {
   svg.addEventListener("pointercancel", onUp);
 }
 
+// Left-click on empty canvas: ambiguous between "deselect" and "pan".
+// We resolve by movement — under the drag threshold m gets the historic
+// "click empties the selection" behaviour; past the threshold the gesture
+// promotes to a pan (Excalidraw / Figma standard). 3px screen-space dead
+// zone is enough that a steady click doesn't accidentally nudge the view.
+const EMPTY_CANVAS_PAN_THRESHOLD_PX = 3;
+
+function startEmptyCanvasGesture(e) {
+  const svg = /** @type {SVGSVGElement} */ ($("#canvas"));
+  const ctm = svg.getScreenCTM();
+  if (!ctm) return;
+  const scaleX = ctm.a, scaleY = ctm.d;
+  const startClientX = e.clientX, startClientY = e.clientY;
+  const startViewX = state.view.x, startViewY = state.view.y;
+  let panning = false;
+  try { svg.setPointerCapture(e.pointerId); } catch {}
+  const onMove = (ev) => {
+    const dx = ev.clientX - startClientX;
+    const dy = ev.clientY - startClientY;
+    if (!panning) {
+      if (Math.hypot(dx, dy) <= EMPTY_CANVAS_PAN_THRESHOLD_PX) return;
+      panning = true;
+      $(".canvas-wrap").classList.add("panning");
+    }
+    state.view.x = startViewX - dx / scaleX;
+    state.view.y = startViewY - dy / scaleY;
+    applyViewBox();
+  };
+  const onUp = (ev) => {
+    svg.removeEventListener("pointermove", onMove);
+    svg.removeEventListener("pointerup",   onUp);
+    svg.removeEventListener("pointercancel", onUp);
+    try { svg.releasePointerCapture(ev.pointerId); } catch {}
+    if (panning) {
+      $(".canvas-wrap").classList.remove("panning");
+      setViewInURL();
+    } else if (state.selection) {
+      state.selection = null;
+      render();
+    }
+  };
+  svg.addEventListener("pointermove", onMove);
+  svg.addEventListener("pointerup",   onUp);
+  svg.addEventListener("pointercancel", onUp);
+}
+
 function resetView() {
   state.view.zoom = 1;
   state.view.x = 0;
@@ -2036,12 +2082,18 @@ function onCanvasPointerDown(e) {
     return;
   }
 
-  // No tool armed: clicks that started on a device/frame/io go to their
-  // own handlers (drag / select). Leave them alone.
-  if (e.target instanceof Element && e.target.closest("[data-device-id], [data-frame-id], [data-io-id]")) return;
+  // No tool armed: clicks that started on a device/frame/io/clamp/port/cable
+  // go to their own handlers (drag / select / replug). Leave them alone.
+  if (e.target instanceof Element && e.target.closest("[data-device-id], [data-frame-id], [data-io-id], [data-clamp-id], [data-port-id], [data-cable-id]")) return;
 
-  // Plain canvas click = clear selection.
-  if (state.selection) { state.selection = null; render(); }
+  // Empty-canvas left-click without an active cable draw: start a
+  // maybe-pan gesture. It promotes to a pan once the cursor crosses the
+  // drag threshold; if m clicks without dragging it falls back to the
+  // historic "clear selection" UX. Other buttons fall through (middle is
+  // already handled above, right-click is the browser context menu).
+  if (e.button === 0 && state.cableDrawFromPortID == null) {
+    startEmptyCanvasGesture(e);
+  }
 }
 
 function startFrameRubberBand(e, p0) {