feat: DEFAULT_FOLDER pre-selects a download folder (closes #875)

Most downloads from a given install land in the same custom directory,
which today means picking it by hand every time. DEFAULT_FOLDER seeds the
folder field once the configuration arrives; the field stays editable, so
per-download folders still work, and a folder already typed this session
is not overwritten.

The value is trimmed of surrounding slashes, and dropped with a warning
when CUSTOM_DIRS is off, since the UI hides the field in that mode and
the download path check rejects a folder anyway.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/app/app.spec.ts

@@ -148,6 +148,25 @@ describe('App', () => {
     expect(app).toBeTruthy();
   });
 
+  it('pre-fills the download folder from DEFAULT_FOLDER', () => {
+    const fixture = TestBed.createComponent(App);
+    fixture.detectChanges();
+
+    downloads.configurationChanged.next({ DEFAULT_FOLDER: 'youtube' });
+
+    expect(fixture.componentInstance.folder).toBe('youtube');
+  });
+
+  it('does not overwrite a folder the user already typed', () => {
+    const fixture = TestBed.createComponent(App);
+    fixture.detectChanges();
+    fixture.componentInstance.folder = 'music';
+
+    downloads.configurationChanged.next({ DEFAULT_FOLDER: 'youtube' });
+
+    expect(fixture.componentInstance.folder).toBe('music');
+  });
+
   it('asIsOrder returns a stable comparator value (insertion order preserved)', () => {
     const fixture = TestBed.createComponent(App);
     const app = fixture.componentInstance;

ui/src/app/app.ts

@@ -434,6 +434,12 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
         if (!Number.isNaN(playlistItemLimit) && playlistItemLimit > 0) {
           this.playlistItemLimit = playlistItemLimit;
         }
+        // Pre-fill the download folder, unless the user has already typed one
+        // this session. The server drops DEFAULT_FOLDER when CUSTOM_DIRS is
+        // off, so there is nothing to guard against here.
+        if (!this.folder) {
+          this.folder = String(config['DEFAULT_FOLDER'] ?? '');
+        }
         // Set chapter template from backend config if not already set by cookie
         if (!this.chapterTemplate) {
           this.chapterTemplate = config['OUTPUT_TEMPLATE_CHAPTER'];