fix(reader): drop "Chapter N:" prefix from chapter title display (0.51.2)

The chapter list on the manga detail page, the reader's chapter-select
dropdown, the continuous-mode chapter bar, the browser tab title, and
the profile upload-history entries all prepended "Chapter {number}:"
in front of the crawled site title. Source titles already include
"Ch.N" themselves and the manga page renders chapters inside an <ol>,
so the prefix duplicated information the user could already see.

A small chapterLabel(c) helper in $lib/api/chapters returns the site
title as-is, falling back to "Chapter {number}" only when the
crawler captured an empty title (link/option stays non-empty). The
five render sites now call it. The previous-/next-chapter nav
buttons still read "Previous chapter (Ch. N)" / "Next chapter (Ch. N)"
since those are wayfinding labels, not title display.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/e2e/reader.spec.ts

@@ -120,7 +120,7 @@ test('manga overview shows title, cover, and a chapter list', async ({ page }) =
     await expect(page.getByTestId('manga-title')).toHaveText('Berserk');
     await expect(page.getByTestId('manga-author')).toContainText('Kentaro Miura');
     await expect(page.getByTestId('manga-cover')).toBeVisible();
-    await expect(page.getByTestId('chapter-list')).toContainText('Chapter 1');
+    await expect(page.getByTestId('chapter-list')).toContainText('The Brand');
     await expect(page.getByTestId('bookmark-signin')).toBeVisible();
 });