web: friendly Paste ID not found page for missing pastes (#173)

Missing or expired paste IDs (and unknown routes) now render the main
UI chrome with a 'Paste ID not found' message in a color-coded result
card instead of a bare text/JSON 404. HTTP status stays 404. No inline
scripts or styles; new CSS uses existing --err token and pill radii.

internal/web/static/app.css

@@ -341,6 +341,9 @@ td a.slug.paste-name { background: none; padding: 0; border-radius: 0; font-fami
 }
 .inner h1 { font-size: 29.2px; font-weight: 600; margin-bottom: 6px; }
 .inner .sub { font-size: 22.4px; color: var(--muted-fg); margin-bottom: 20px; }
+/* #173: missing paste message box */
+.notfound-err { color: var(--err); }
+.notfound-card .btn { display: block; margin-top: 10px; }
 .pwinput {
   width: 100%; padding: 10px 14px; border: 1px solid var(--border); border-radius: var(--radius);
   background: var(--bg); color: var(--fg); font: inherit; font-size: 23.2px; outline: none; text-align: center;