Add only what is a feed or links one; refuse the rest (#102)

Adding an address looked for the feed a web page links and, finding none, added the address
as it was: every check then failed, and the sidebar called it a feed that had moved. cnn.com
is one; its page links no feed. find_feed replaces feed_behind_page: the address is added if
it is a feed or an OPML list, the feed its page links if it is a web page that links one (and
that is a feed), and otherwise the add is refused with the reason, from the web page (400, the
dialog stays open) and from `ipx add`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ui/app.spec.js

@@ -927,6 +927,19 @@ test('adding a feed scans it straight away', async ({ page }) => {
   await expect(page.locator('.ep', { hasText: 'Fresh Ep' })).toBeVisible({ timeout: 10_000 });
 });
 
+test('adding a page adds the feed it links, and a page with no feed is refused', async ({ page }) => {
+  const feeds = await page.locator('.feed').count();
+  await page.locator('#addFeed').click();
+  await page.locator('#nurl').fill('http://127.0.0.1:8792/nofeed.html');
+  await page.locator('#nsave').click();
+  await expect(page.locator('.toast')).toContainText('links no feed');
+  await expect(page.locator('#modal.on')).toBeVisible();             // left open to correct it
+  await expect(page.locator('.feed')).toHaveCount(feeds);
+  await page.locator('#nurl').fill('http://127.0.0.1:8792/site.html');
+  await page.locator('#nsave').click();
+  await expect(page.locator('.feed', { hasText: 'Linked Site' })).toBeVisible({ timeout: 20_000 });
+});
+
 test('a deleted file looks as if it was never downloaded', async ({ page }) => {
   // Other people subscribe to Picture Blog by now, so both prompts come; take them.
   page.on('dialog', d => d.accept());

tests/ui/fixtures/linked.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<rss version="2.0"><channel><title>Linked Site</title><link>http://127.0.0.1:8792/site.html</link>
+<item><title>Linked Post</title><guid>linked-1</guid></item>
+</channel></rss>

tests/ui/fixtures/nofeed.html

@@ -0,0 +1,2 @@
+<!doctype html>
+<html><head><title>No Feed Here</title></head><body>A site that links no feed.</body></html>

tests/ui/fixtures/site.html

@@ -0,0 +1,4 @@
+<!doctype html>
+<html><head><title>A Site</title>
+<link rel="alternate" type="application/rss+xml" href="/linked.xml">
+</head><body>A site with a feed.</body></html>