Add page transition animations and background wallpapers

Wrap page views in {#key} blocks with a fade-in animation for smooth
transitions. Add Demon Slayer wallpapers as fixed pseudo-element
backgrounds on main (Tanjiro for dashboard, Rengoku for accelerator)
with CSS crossfade. Fix section spacing lost by the page-enter wrapper
div. Minor tooltip text fixes.

ui/src/lib/AcceleratorPage.svelte

@@ -21,6 +21,7 @@
   let boostedList = $state<BoostedTx[]>([]);
   let loadingMax = $state(false);
 
+
   const feerate = $derived(parseFloat(feerateInput) || 0);
   const validTxid = $derived(/^[0-9a-fA-F]{64}$/.test(txid.trim()));
   const overCap = $derived(feerate > 2000);
@@ -310,8 +311,10 @@
   </section>
 </section>
 
+
 <style>
   .page {
+    position: relative;
     display: flex;
     flex-direction: column;
     gap: 1.5rem;
@@ -406,13 +409,18 @@
 
   /* ── Boost form ── */
   .boost-form {
+    position: relative;
+    overflow: hidden;
     padding: 2rem;
   }
   .boost-form h3 {
+    position: relative;
+    z-index: 1;
     margin: 0 0 1.5rem;
     font-size: 1.3rem;
     font-weight: 700;
   }
+
   .field {
     margin-bottom: 1.5rem;
   }