refactor(frontend): ApiError interface, tempIdSuffix rename, NET_GRID constants, extract onPaletteDrop handlers

ApiError: defined once in utils/api.ts, replaces 9 ad-hoc anonymous casts
across MazeNET, Inspector, DeckyFleet, SwarmHosts, Webhooks, PersonaGeneration,
ServiceConfigFields, CanaryTokens.

hex4 renamed to tempIdSuffix — the name now matches the comment that already
explained its purpose.

NET_GRID_{W,H,GAP,COLS} extracted from inline magic numbers to module-level
constants in MazeNET.tsx.

onPaletteDrop (130-line useCallback) split into three module-level handlers
(_dropNetwork, _dropArchetype, _dropService); the callback becomes a 10-line
router.

decnet_web/src/utils/api.ts

@@ -24,3 +24,9 @@ api.interceptors.response.use(
 );
 
 export default api;
+
+/** Shape of an axios error response from the DECNET API. */
+export interface ApiError {
+  response?: { status?: number; data?: { detail?: string } };
+  message?: string;
+}