Redesign the feed settings dialog (#167)

One column of label, box and grey paragraph, eight times at the same weight, three screens long on a
phone with Save at the bottom of the third. Approved from the mockups on #167.

- Download automatically comes first, as a switch; how many, the keywords and explicit, which only
  ever applied to automatic downloads, hang beneath it and fold away when it is off.
- How many is Default, Newest or All, said as a sentence with the number in it, over a strip of the
  feed's own newest items, each as tall as it is long and lit where it downloads; dragging along
  it sets Newest. All is 0, which now means all (#166).
- Keywords and hidden words are chips: Enter or a comma adds one, its x or Backspace removes it.
- The feed's own settings, the same for everyone and an admin's to change, sit apart in a box.
- The body scrolls and Cancel and Save stay at the foot.
- Theme colours only; iOS switches, but checkboxes in Classic. openModal takes a class for a
  dialog with a frame of its own, set whole each time so it never outlives it.

A browser test covers the strip, chips, the fold and a save that holds.

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

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- A feed's settings are redesigned: downloading on its own is a switch with what depends on it beneath, how many is Default, Newest or All shown on a strip of the show's own episodes, words to pick or hide are chips, the settings shared by everyone sit apart, and Save is always in reach.
 - The feed list's traces show each of its database queries, so it can be told where its time goes.
 - Saving a feed's settings asks the database for your subscription once, not twice.
 - `ipx add` and Add feed in the page add a feed the same way, through one piece of code.