setup: flex-ify, light/dark, keep page position on reload

got rid of a bunch of m[l/r/x/y]-x tailwind classes and used more
flex-[row/col] gap-2's. UI should be more consistent in general, and
with the admin UI.

The page you were on is actually read from the URL on reload, however
does not keep settings (implemented just for ease of UI editing,
really).

`missing-colors.js` preprocessor script now applies dark prefixes for
<section>s, but like with cards, does not apply a default ~neutral to
those without, so that <section class=""> looks different to <section
class="~neutral">.

Light/dark selector added to setup too, and the actual mode given to the
browser through CSS `color-scheme` is correct, meaning things like textareas, checkboxes and
controls are now colored according to the theme.

lang/setup/en-us.json

@@ -102,6 +102,7 @@
     "jellyseerr": {
         "title": "Jellyseerr",
         "description": "Jellyseerr is an alternative to Ombi, and integrates with jfa-go slightly better. Again, after setup is finished, go to Settings to create a profile and add a template for new Jellyseerr accounts.",
+        "importExisting": "Import existing users",
         "importExistingDescription": "If enabled, your existing users will have contact details and preferences from jfa-go synchronized."
     },
     "messages": {
@@ -164,6 +165,7 @@
     "helpMessages": {
         "title": "Help Messages",
         "description": "These messages will display in the account creation page and in some emails.",
+        "markdownMessageNotice": "Contents of some emails, pages and messages can be customized further with markdown in Settings.",
         "contactMessage": "Contact Message",
         "contactMessageNotice": "Displays at the bottom of all pages except admin.",
         "helpMessage": "Help Message",