Update Mihomo documentation and proxy handling for improved error debugging

- Enhanced `GATEWAY_RUN.md` with detailed debugging steps for handling **400** errors in Mihomo routes, clarifying the configuration requirements and common pitfalls.
- Updated comments in `internal/proxy/mihomo.go` to reflect changes in error handling and proxy behavior, linking to the new debugging section in the documentation.
- Modified the Svelte component logic to ensure proper filtering of selectable groups, improving the user interface for managing Mihomo proxies.

web/src/routes/servers/[alias]/mihomo/+page.svelte

@@ -202,8 +202,12 @@
 		void loadProxies();
 	});
 
-	function isSelectableGroup(t?: string) {
-		return t === 'Selector' || t === 'URLTest';
+	/** Группы, у которых есть `all` + `now` и PUT /proxies/{name} переключает узел (как в Clash/Mihomo). */
+	function isSelectableGroup(p: MihomoProxyEntry) {
+		const t = (p.type ?? '').toLowerCase();
+		if (t === 'selector' || t === 'urltest' || t === 'fallback') return true;
+		if (t === 'loadbalance' && Array.isArray(p.all) && p.all.length > 0) return true;
+		return false;
 	}
 
 	function lastDelay(p: MihomoProxyEntry): number | null {
@@ -386,7 +390,7 @@
 		{#snippet children()}
 			{#if proxiesData?.proxies}
 				<div class="flex flex-col gap-8">
-					{#each Object.entries(proxiesData.proxies).filter(([_, v]) => isSelectableGroup(v.type)) as [gName, group] (gName)}
+					{#each Object.entries(proxiesData.proxies).filter(([_, v]) => isSelectableGroup(v)) as [gName, group] (gName)}
 						<div class="space-y-3">
 							<div class="flex flex-wrap items-center justify-between gap-2">
 								<div>