feat(threads): add composable to get current threadId

- in main app: based on router, casting to number
- in sidebar: internal value

Signed-off-by: Maksim Sukharev <antreesy.web@gmail.com>

src/composables/useGetThreadId.ts

@@ -0,0 +1,27 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { createSharedComposable } from '@vueuse/core'
+import { computed, ref } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+
+/**
+ * Shared composable to get threadId of current thread in conversation
+ */
+export const useGetThreadId = createSharedComposable(function() {
+	const router = useRouter()
+	const route = useRoute()
+
+	if (router) {
+		return computed<number>({
+			get: () => route.query.threadId ? Number(route.query.threadId) : 0,
+			set: (value: number) => {
+				router.push({ query: { ...route.query, threadId: value !== 0 ? value : undefined } })
+			},
+		})
+	} else {
+		return ref(0)
+	}
+})