Remove duplicates in the web interface

The settings page has one Save button helper. The story list sorts by
the sort key that the server sends, so the rule for a leading article
exists once. The story page takes its list of download modes from the
mode hints.

tests/js/story-list.test.mjs

@@ -16,8 +16,11 @@ const { storiesPage } = await import('../../bindery/static/js/stories.js');
 const { refreshJobs, startPolling, stopPolling } = await import('../../bindery/static/js/jobs.js');
 const { fmtDate } = await import('../../bindery/static/js/lib.js');
 
+// The server gives each story its sort key: the title in lower case, without a leading article.
 const story = (id, title, changes = {}) => ({
-  id, title, url: `https://example.com/${id}`, author: 'Reader', site: 'example.com',
+  id, title, url: `https://example.com/${id}`,
+  sort_title: (title || `https://example.com/${id}`).trim().replace(/^(?:the|an?)\s+/i, '').toLowerCase(),
+  author: 'Reader', site: 'example.com',
   chapter_count: 42, offsite_count: 0, revised_count: 0, status: 'ok', message: 'No changes',
   last_changed: null, last_checked: null, ...changes,
 });