#173: render friendly 'Paste ID not found' UI instead of bare 404

Missing/expired/burned paste IDs and unknown routes now render the main
UI (topbar + centered result card) with a 'Paste ID not found' message,
returning HTTP 404 status for correctness.

internal/web/templates/notfound.html

@@ -0,0 +1,13 @@
+{{template "head" .}}
+{{template "topbar" .}}
+<div class="center">
+  <div class="float notfound-card">
+    <div class="inner">
+      <div class="lockring"><svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="12" cy="12" r="10"/><line x1="12" y1="8" x2="12" y2="12"/><line x1="12" y1="16" x2="12.01" y2="16"/></svg></div>
+      <h1>Paste ID not found</h1>
+      <p class="sub">The paste <span class="slug">/{{.ID}}</span> does not exist, has expired, or was burned.</p>
+      <a class="btn" href="/new">Create a new paste</a>
+    </div>
+  </div>
+</div>
+{{template "foot" .}}