refactor(frontend): Split friends.ts store into focused modules

Splits the 806-line friends.ts into three focused files:
- friends.ts (194 lines) — core friend CRUD, derived stores, re-exports
- friendSubresources.ts (542 lines) — all subresource CRUD operations
- friendListFilter.ts (94 lines) — filter/search state persistence

All existing imports from '$lib/stores/friends' continue to work
unchanged via re-exports. No consumer changes needed.

Closes #113

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: enko

apps/frontend/src/lib/stores/friendListFilter.ts

@@ -0,0 +1,94 @@
+import { derived, writable } from 'svelte/store';
+import type { FacetFilters } from '$shared';
+
+interface FriendListFilterState {
+  query: string;
+  filters: FacetFilters;
+}
+
+const initialFilterState: FriendListFilterState = {
+  query: '',
+  filters: {},
+};
+
+function createFriendListFilterStore() {
+  const { subscribe, set, update } = writable<FriendListFilterState>(initialFilterState);
+
+  return {
+    subscribe,
+
+    setQuery: (query: string) => {
+      update((state) => ({ ...state, query }));
+    },
+
+    setFilters: (filters: FacetFilters) => {
+      update((state) => ({ ...state, filters }));
+    },
+
+    setState: (query: string, filters: FacetFilters) => {
+      set({ query, filters });
+    },
+
+    clear: () => {
+      set(initialFilterState);
+    },
+
+    buildSearchParams: (state: FriendListFilterState): URLSearchParams => {
+      const params = new URLSearchParams();
+
+      if (state.query.trim()) {
+        params.set('q', state.query.trim());
+      }
+
+      for (const [key, values] of Object.entries(state.filters)) {
+        if (values && values.length > 0) {
+          params.set(key, values.join(','));
+        }
+      }
+
+      return params;
+    },
+
+    parseSearchParams: (params: URLSearchParams): FriendListFilterState => {
+      const query = params.get('q') ?? '';
+      const filters: FacetFilters = {};
+
+      const stringArrayKeys = [
+        'country',
+        'city',
+        'organization',
+        'job_title',
+        'department',
+        'circles',
+      ] as const;
+      for (const key of stringArrayKeys) {
+        const value = params.get(key);
+        if (value) {
+          filters[key] = value.split(',');
+        }
+      }
+
+      const relationshipCategory = params.get('relationship_category');
+      if (relationshipCategory) {
+        filters.relationship_category = relationshipCategory.split(',') as (
+          | 'family'
+          | 'professional'
+          | 'social'
+        )[];
+      }
+
+      return { query, filters };
+    },
+  };
+}
+
+export const friendListFilter = createFriendListFilterStore();
+
+export const friendListQuery = derived(friendListFilter, ($filter) => $filter.query);
+
+export const hasFriendListFilters = derived(
+  friendListFilter,
+  ($filter) =>
+    $filter.query.trim().length > 0 ||
+    Object.values($filter.filters).some((arr) => arr && arr.length > 0),
+);