Move the app's markup to the D components and Tailwind 4

The pages and components now use the daisyUI wrappers and the neo-* utilities
instead of Bulma and the neomorph classes:
- buttons are DButton, with pressed states as Active;
- nav links are btn / btn-active;
- the filter and composer selects are DSelect with a prefix icon;
- the composer's fields are DInput and DTextarea;
- the message options and scope pickers are DDropdown, which now close on an
  outside click and on Escape;
- the settings and admin sections are DCollapse;
- the dark-mode switch is a DToggle with sun and moon icons;
- the hue slider is a DRange on the rainbow track.

DropdownButton, OpenDownContainer and LoadingData are gone. The loader is
DLoading, whose animation runs; the old one's keyframes were never defined.
Icons coloured with Bulma's has-text-* now get daisyUI's text-info, success,
warning and error, which had no rules before.

Tailwind 3 to 4 fixes:
- space-x/y become gap, which also handles the flex-col-reverse feeds;
- !rounded-lg becomes rounded-lg!;
- drop-shadow becomes drop-shadow-sm;
- text-md becomes text-base.
The thread page marks its message with Content's new Highlighted parameter
rather than a second shadow size.

neo.css: a select's prefix icon gets the room daisyUI leaves for a span.label,
and the inner select lets the well show through.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Pages/Index.razor

@@ -4,14 +4,17 @@
 @inherits LocalizableComponentBase
 <Title>@Localizer["Index"]</Title>
 
-<div class="flex w-full h-full flex-col space-y-4">
+<div class="flex w-full h-full flex-col gap-4">
 
-	<section class="block relative w-full h-full neomorphInset is-nxsmall rounded-xl">
-		<div class="flex flex-col space-y-4 p-4 md:p-5 w-full h-full absolute overflow-y-auto @SUtility.IfTrueThen(TimeSortingType is TimeSortingType.Descending, "flex-col-reverse")">
+	<section class="block relative w-full h-full neo-inset neo-xs rounded-xl">
+		<div class="flex flex-col gap-4 p-4 md:p-5 w-full h-full absolute overflow-y-auto @SUtility.IfTrueThen(TimeSortingType is TimeSortingType.Descending, "flex-col-reverse")">
 
 			@if (IsLoading)
 			{
-				<LoadingData />
+				<p class="flex w-full items-center justify-center gap-2">
+					<DLoading Variant="DLoadingVariant.Dots" LabelText="@Localizer["loading"]" />
+					@Localizer["loading"]
+				</p>
 			}
 			else if (Messages.Count == 0)
 			{