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/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"
                     );