feat(ui): double-click a port to start a cable draw

Double-click a port → enter cable-draw mode from that port without
having to arm the cable tool first. armTool("cable") is called so
the crosshair cursor is active during the draw; the next port-click
hits the existing cable-draw-in-progress branch in onPortPointerDown
and commits the cable. Esc / clicking the source port cancels.

Single-click behaviour (select + open port inspector) is unchanged
because pointerdown still hits onPortPointerDown first; dblclick
upgrades the selection to a cable-draw source.

web/static/main.js

@@ -471,6 +471,18 @@ function renderCanvas() {
       });
       // Port-click drives both cable-draw (slice 7) and port-select (this fix).
       c.addEventListener("pointerdown", (e) => onPortPointerDown(e, prt));
+      // Double-click activates cable-draw mode from this port without arming
+      // the cable tool first. armTool("cable") gives the crosshair cursor;
+      // the next port-click is then caught by onPortPointerDown's
+      // cable-draw-in-progress branch and commits the cable.
+      c.addEventListener("dblclick", (e) => {
+        e.stopPropagation();
+        e.preventDefault();
+        if (state.tool !== "cable") armTool("cable");
+        state.cableDrawFromPortID = prt.id;
+        state.selection = null;
+        render();
+      });
       g.append(c);
     }