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/components/ServiceConfigFields.tsx

@@ -1,5 +1,5 @@
 import React, { useEffect, useMemo, useState } from 'react';
-import api from '../utils/api';
+import api, { type ApiError } from '../utils/api';
 import './ServiceConfigForm.css';
 
 export interface ServiceConfigFieldDTO {
@@ -57,7 +57,7 @@ export function compactPayload(
 }
 
 export const fmtSchemaError = (err: unknown, fallback: string): string =>
-  (err as { response?: { data?: { detail?: string } } })?.response?.data?.detail
+  (err as ApiError)?.response?.data?.detail
   ?? fallback;
 
 interface Props {