style(design): consistency sweep + UX upgrades

- Palette: gray/zinc/slate/neutral → stein, red → fire, amber → erde,
  sky → himmel, green → wald
- Footer: replaced raw rgb with stein tokens
- Radius: unified to rounded-xs across cards, buttons, inputs, pagination
- Buttons: .btn-primary/.btn-secondary now font-medium; added secondary
- Card hover: -translate-y-0.5 + shadow + wald-300 border
- Header active link: wald border-bottom + wald-700 text
- Mobile nav: bigger touch targets + wald active accent
- Pagination: prev/next text labels + wald-500 active state with
  separated shape/color classes to avoid Tailwind conflicts
- Lead paragraph: scoped .markdown > p:first-child for top-level
  MarkdownBlock only via [data-block-type="markdown"]
- Section dividers: subtle wald-200 gradient between content rows
- Global focus-visible ring (wald-500)
- Inline hex → palette tokens (Badge amber, Tag custom-color contrast)
- Font weights snapped to design system (300/400/500/600/700)
- transition-all → transition-colors where only color changes
- Removed em-dashes from user-visible templates

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/blocks/YoutubeVideoGalleryBlock.svelte

@@ -169,7 +169,7 @@
         <li class="m-0 p-0">
           <button
             type="button"
-            class="group relative block w-full aspect-video overflow-hidden rounded-sm bg-stein-100 cursor-pointer focus:outline-none focus:ring-2 focus:ring-wald-500"
+            class="group relative block w-full aspect-video overflow-hidden rounded-xs bg-stein-100 cursor-pointer focus:outline-none focus:ring-2 focus:ring-wald-500"
             onclick={() => openModal(i)}
             aria-label={label || t(T.youtube_title_fallback)}
           >
@@ -222,7 +222,7 @@
           onclick={(e) => e.stopPropagation()}
           role="presentation"
         >
-          <div class="aspect-video w-full overflow-hidden rounded-sm bg-black">
+          <div class="aspect-video w-full overflow-hidden rounded-xs bg-black">
             {#key modalIndex}
               <iframe
                 title={curLabel || t(T.youtube_title_fallback)}
@@ -280,7 +280,7 @@
         <div class="grid grid-cols-1 gap-4 md:grid-cols-2">
           {#key currentIndex}
             {@const current = items[currentIndex]}
-            <div class="aspect-video w-full overflow-hidden rounded-sm bg-stein-100" transition:fade={{ duration: 150 }}>
+            <div class="aspect-video w-full overflow-hidden rounded-xs bg-stein-100" transition:fade={{ duration: 150 }}>
               <iframe
                 title={current.video.title ?? t(T.youtube_title_fallback)}
                 src={current.embedUrl}
@@ -291,7 +291,7 @@
               ></iframe>
             </div>
             {#if second}
-              <div class="hidden md:block aspect-video w-full overflow-hidden rounded-sm bg-stein-100" transition:fade={{ duration: 150 }}>
+              <div class="hidden md:block aspect-video w-full overflow-hidden rounded-xs bg-stein-100" transition:fade={{ duration: 150 }}>
                 <iframe
                   title={second.video.title ?? t(T.youtube_title_fallback)}
                   src={second.embedUrl}