feat: shared loading state + cleanup loading state management (#5835)

* feat: implement shared loading bar component and polished loading states across the app

* feat: align loading states + ensureQueryData changes

* fix: lint + bugs

* fix: skeleton for manage servers page

* fix: merge conflict fix

apps/frontend/src/providers/setup/loading-state.ts

@@ -0,0 +1,49 @@
+import type { LoadingStateProvider } from '@modrinth/ui'
+import { createLoadingStateCore, provideLoadingState } from '@modrinth/ui'
+import { watch } from 'vue'
+
+/**
+ * Initialize the cross-platform loading-state provider for the website.
+ *
+ * Responsibilities:
+ * 1. Own the token-based ref-counter that drives `LoadingBar` and `ReadyTransition`.
+ * 2. Bridge the legacy `useState('loading')` global so the many existing
+ *    `startLoading()` / `stopLoading()` call sites continue to raise the bar.
+ * 3. Register Nuxt `page:start` / `page:finish` hooks so route navigation
+ *    auto-fires the bar (replaces the behavior previously inside
+ *    `modrinth-loading-indicator.ts`).
+ */
+export function setupLoadingStateProvider(): LoadingStateProvider {
+	const provider = createLoadingStateCore({ barEnabled: true })
+	provideLoadingState(provider)
+
+	const legacyState = useLoading()
+	let legacyToken: symbol | null = null
+	watch(
+		legacyState,
+		(value) => {
+			if (value && !legacyToken) {
+				legacyToken = provider.begin()
+			} else if (!value && legacyToken) {
+				provider.end(legacyToken)
+				legacyToken = null
+			}
+		},
+		{ immediate: true },
+	)
+
+	const nuxtApp = useNuxtApp()
+	let pageToken: symbol | null = null
+	nuxtApp.hook('page:start', () => {
+		if (pageToken) provider.end(pageToken)
+		pageToken = provider.begin()
+	})
+	nuxtApp.hook('page:finish', () => {
+		if (pageToken) {
+			provider.end(pageToken)
+			pageToken = null
+		}
+	})
+
+	return provider
+}