cable-sizing: fix trigger column and add visible affordance

The trigger was wired to the cableCrossSection column, which is not in
the default visible column set - the actually visible "Kabel" column
is cableSummary (a computed cableType/cableCrossSection/cableLength
join). Editing cableSummary already maps to the remark field rather
than the underlying cable data (see buildCircuitEditPatch), so this
column was effectively a read-only display already; retargeting the
trigger there loses no existing manual-edit capability. Also adds
.cell-cable-sizing-trigger styling (mirrors .cell-protection-trigger)
so the trigger is visually discoverable, which the initial version was
missing entirely.

src/app/globals.css

@@ -1209,6 +1209,30 @@ a.kpi:hover {
   box-shadow: inset 0 0 0 1px var(--color-signal);
 }
 
+.tree-grid .cell-cable-sizing-trigger {
+  color: var(--color-primary);
+  cursor: pointer;
+  font-weight: 600;
+  transition:
+    background-color 0.12s ease,
+    box-shadow 0.12s ease;
+}
+
+:root[data-bs-theme="dark"] .tree-grid .cell-cable-sizing-trigger {
+  color: var(--color-accent-pale);
+}
+
+.tree-grid .cell-cable-sizing-trigger:hover {
+  background: rgba(63, 130, 166, 0.15);
+  box-shadow: inset 0 0 0 1px var(--color-primary);
+}
+
+.tree-grid .cell-cable-sizing-trigger::after {
+  content: " ⚡";
+  font-size: 0.8em;
+  opacity: 0.7;
+}
+
 .tree-grid .device-drag-handle {
   cursor: grab;
 }

src/frontend/components/circuit-tree-editor.tsx

@@ -4277,7 +4277,7 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                         row.rowType !== "deviceRow"
                     );
                     const isCableSizingTrigger = Boolean(
-                      column.key === "cableCrossSection" &&
+                      column.key === "cableSummary" &&
                         row.circuit &&
                         row.rowType !== "deviceRow"
                     );