Browse the Directory by category, then subcategory, as Apple does (#118)

The Directory had one row of chips holding whatever each feed's category was, a category
(Technology) or a subcategory (Tech News, Video Games) side by side: a podcast's own
<itunes:category> is stored as its subcategory, and Jev's answers (#117) are often
subcategories too, so after the first forced scan the row held 22 chips. /api/directory now
gives each feed's Apple category and subcategory, worked out from Apple's list, and the
page shows the categories, then a picked one's subcategories on a line of their own. A
category that is not Apple's, one an admin typed, stands as a category of its own.

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

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The Directory browses as Apple's does: a row of categories, and once one is picked, a row of its subcategories (Tech News under News, Video Games under Leisure), instead of one row mixing both.
 - An item published without a title shows its opening words, in plain text rather than bold, instead of "(untitled)"; one with no text either shows its file's name, or its show and date. Opened, it starts with its text.
 - A feed that has moved for good (a permanent redirect) is followed to its new address, which iPX then reads from, and says so in the log as `feed_moved`. A temporary redirect changes nothing.
 - The daemon sleeps until the next feed is due, at most ten minutes, instead of looking every minute; refreshing or adding a feed still wakes it at once.