Fix library filter bar button misalignment

.field's margin-bottom (meant for vertically-stacked fields) pushed each
field's box lower than the bare Filter button under align-items: flex-end,
making the button look higher than the search/playlist/format boxes next
to it. New .filter-bar class zeroes that margin instead of the previous
ad-hoc inline flex styling.

app/templates/library/index.html

@@ -12,7 +12,7 @@
 <div class="notice success">Track deleted.</div>
 {% endif %}
 
-<form method="get" action="/library" class="panel" style="display:flex; gap:0.75rem; flex-wrap:wrap; align-items:flex-end; max-width:none;">
+<form method="get" action="/library" class="panel filter-bar">
   <div class="field">
     <label for="q">Search</label>
     <input type="text" id="q" name="q" value="{{ q }}" placeholder="artist, title, album...">