feat(settings): modernize download location settings

- Replace split 'Default Download Path' and 'All Categories Base' with a canonical baseDownloadFolder and categorySubfolders model
- Retain absolute category overrides only where required
- Update Add window to accurately reflect intended destination:
  - Single URL: specific category path
  - Multiple URLs: base folder with explanatory text
  - Manual Browse override: selected folder
- Centralize path resolution logic shared by Rust backend and UI

src/utils/downloadLocations.test.ts

@@ -0,0 +1,65 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('@tauri-apps/api/path', () => ({
+  join: vi.fn(async (...parts: string[]) =>
+    parts
+      .map((part, index) => index === 0 ? part.replace(/[\\/]+$/, '') : part.replace(/^[\\/]+|[\\/]+$/g, ''))
+      .join('/')
+  )
+}));
+
+import {
+  DEFAULT_CATEGORY_SUBFOLDERS,
+  normalizeCategorySubfolder,
+  normalizeDownloadLocationSettings,
+  resolveCategoryDestination
+} from './downloadLocations';
+
+describe('download locations', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('migrates legacy derived directories without creating overrides', () => {
+    const settings = normalizeDownloadLocationSettings({
+      defaultDownloadPath: '/Users/test/Downloads',
+      downloadDirectories: {
+        Movies: '/Users/test/Downloads/Movies',
+        Documents: '/Users/test/Downloads/Documents'
+      }
+    });
+
+    expect(settings.baseDownloadFolder).toBe('/Users/test/Downloads');
+    expect(settings.categorySubfolders).toEqual(DEFAULT_CATEGORY_SUBFOLDERS);
+    expect(settings.categoryDirectoryOverrides).toEqual({});
+  });
+
+  it('preserves legacy custom category directories as overrides', () => {
+    const settings = normalizeDownloadLocationSettings({
+      defaultDownloadPath: '/Users/test/Downloads',
+      downloadDirectories: {
+        Video: '/Volumes/Media/Movies'
+      }
+    });
+
+    expect(settings.categoryDirectoryOverrides.Movies).toBe('/Volumes/Media/Movies');
+  });
+
+  it('resolves automatic and overridden category destinations', async () => {
+    const automatic = normalizeDownloadLocationSettings({
+      baseDownloadFolder: '/Users/test/Downloads',
+      categorySubfolders: { Movies: 'Video Files' }
+    });
+    expect(await resolveCategoryDestination(automatic, 'Movies'))
+      .toBe('/Users/test/Downloads/Video Files');
+
+    automatic.categoryDirectoryOverrides.Movies = '/Volumes/Media';
+    expect(await resolveCategoryDestination(automatic, 'Movies')).toBe('/Volumes/Media');
+  });
+
+  it('keeps category subfolders relative and permits nested folders', () => {
+    expect(normalizeCategorySubfolder('../Media/./Movies', 'Movies')).toBe('Media/Movies');
+    expect(normalizeCategorySubfolder('C:\\Media\\Movies', 'Movies')).toBe('Media/Movies');
+    expect(normalizeCategorySubfolder('../../', 'Movies')).toBe('Movies');
+  });
+});