feat: let subscriptions be renamed from the list (#1044)

A subscription's name is captured once at subscribe time from the feed's own
title, so playlists — particularly the UULF-prefixed channel-uploads playlists
— all come back named "Videos" and stay that way. Adds an inline editor on the
Name cell, mirroring the existing title-filter edit next to it.

The update route already whitelisted `name` and update_subscription already
applied it, so this is mostly the missing UI. The backend side is validation:
the old `str(changes["name"])` accepted any type, any length and any
whitespace, for a value that is persisted and broadcast to every connected
client. validate_subscription_name now requires a string, collapses interior
whitespace to keep the label single-line, and caps it at 200 characters.

The name is display-only — it is used for the subscription list and log lines,
never for download paths — so renaming cannot move where files land.

ui/src/app/app.spec.ts

@@ -81,7 +81,10 @@ class SubscriptionsServiceStub {
     return of({});
   }
 
-  update() {
+  updateCalls: [string, unknown][] = [];
+
+  update(id: string, changes: unknown) {
+    this.updateCalls.push([id, changes]);
     return of({ status: 'ok' as const });
   }
 
@@ -315,4 +318,37 @@ describe('App', () => {
     expect(errorSpy).toHaveBeenCalledWith('Invalid subscription title filter (regex)');
     errorSpy.mockRestore();
   });
+
+  it('renames a subscription and closes the inline editor', () => {
+    const fixture = TestBed.createComponent(App);
+    const app = fixture.componentInstance;
+    const subs = TestBed.inject(SubscriptionsService) as unknown as SubscriptionsServiceStub;
+
+    app.beginEditName('sub1', 'Videos');
+    expect(app.editingNameId).toBe('sub1');
+    expect(app.nameEditDraft).toBe('Videos');
+
+    app.nameEditDraft = '  Jane uploads  ';
+    app.saveName('sub1');
+
+    expect(subs.updateCalls).toEqual([['sub1', { name: 'Jane uploads' }]]);
+    expect(app.editingNameId).toBeNull();
+  });
+
+  it('blocks renaming a subscription to an empty name', () => {
+    const toasts = TestBed.inject(ToastService);
+    const errorSpy = vi.spyOn(toasts, 'error').mockImplementation(() => undefined);
+    const fixture = TestBed.createComponent(App);
+    const app = fixture.componentInstance;
+    const subs = TestBed.inject(SubscriptionsService) as unknown as SubscriptionsServiceStub;
+
+    app.beginEditName('sub1', 'Videos');
+    app.nameEditDraft = '   ';
+    app.saveName('sub1');
+
+    expect(subs.updateCalls.length).toBe(0);
+    expect(app.editingNameId).toBe('sub1');
+    expect(errorSpy).toHaveBeenCalledWith('Subscription name must not be empty');
+    errorSpy.mockRestore();
+  });
 });