Add `--page-spacing` variable, fix admin dashboard notice (#30302)

Fixes https://github.com/go-gitea/gitea/issues/30293 and introduce the
`--page-spacing` variable which holds the spacing between the elements
on the page. This is working vertically for all pages, including ones
that have fomantic grid, and horizontally for all that use
`flex-container`.

The `.page-content > :first-child:not(.secondary-nav)` selector uses
margin which in some cases enables to adjacent margins to overlap, which
is nice.

<img width="1320" alt="Screenshot 2024-04-06 at 01 35 19"
src="https://github.com/go-gitea/gitea/assets/115237/3e81e707-e9ff-4b7f-a211-3d98f4f85353">
---
<img width="1327" alt="Screenshot 2024-04-06 at 01 35 45"
src="https://github.com/go-gitea/gitea/assets/115237/aad196c0-9e21-4c06-ae59-7e33a76c61e1">
---
<img width="1321" alt="Screenshot 2024-04-06 at 01 35 31"
src="https://github.com/go-gitea/gitea/assets/115237/785f6c5d-08b6-4e66-aa16-aeca7cfed3ad">

(cherry picked from commit 019857a7015cae32c12b5eac0b895c05f0264b77)

templates/user/notification/notification_div.tmpl

@@ -1,7 +1,7 @@
 <div role="main" aria-label="{{.Title}}" class="page-content user notification" id="notification_div" data-sequence-number="{{.SequenceNumber}}">
 	<div class="ui container">
 		{{$notificationUnreadCount := call .NotificationUnreadCount}}
-		<div class="tw-flex tw-items-center tw-justify-between tw-mb-4">
+		<div class="tw-flex tw-items-center tw-justify-between tw-mb-[--page-spacing]">
 			<div class="small-menu-items ui compact tiny menu">
 				<a class="{{if eq .Status 1}}active {{end}}item" href="{{AppSubUrl}}/notifications?q=unread">
 					{{ctx.Locale.Tr "notification.unread"}}

web_src/css/base.css

@@ -25,6 +25,7 @@
   --min-height-textarea: 132px; /* padding + 6 lines + border = calc(1.57142em + 6lh + 2px), but lh is not fully supported */
   --tab-size: 4;
   --checkbox-size: 16px; /* height and width of checkbox and radio inputs */
+  --page-spacing: 16px; /* space between page elements */
 }
 
 :root * {
@@ -594,11 +595,14 @@ img.ui.avatar,
   margin-bottom: 14px;
 }
 
-/* add padding to all content when there is no .secondary.nav. this uses padding instead of
+/* add margin to all pages when there is no .secondary.nav */
-   margin because with the negative margin on .ui.grid we would have to set margin-top: 0,
-   but that does not work universally for all pages */
 .page-content > :first-child:not(.secondary-nav) {
-  padding-top: 14px;
+  margin-top: var(--page-spacing);
+}
+/* if .ui.grid is the first child the first grid-column has 'padding-top: 1rem' which we need
+   to compensate here */
+.page-content > :first-child.ui.grid {
+  margin-top: calc(var(--page-spacing) - 1rem);
 }
 
 .ui.pagination.menu .active.item {

web_src/css/modules/flexcontainer.css

@@ -2,7 +2,8 @@
 
 .flex-container {
   display: flex !important;
-  gap: 16px;
+  gap: var(--page-spacing);
+  margin-top: var(--page-spacing);
 }
 
 .flex-container-nav {