fix(v3): topbar stacks above the Songs filter bar (instrument/tuner dropdowns)

The v3 Songs filter/search bar (added by #857) is `sticky top-0 z-20 backdrop-blur`
— the SAME z-index and stacking-context recipe as the global topbar, and being
later in source order it won the z-20 tie and painted over the topbar's whole
subtree. So the topbar's instrument-selector dropdown (`data-inst-menu`, z-50
*within* the topbar's z-20 context) rendered BEHIND the filter bar.

Bump the topbar to z-30 so it (and its dropdowns) stack above the per-screen
filter bar. `.z-30` is already in the prebuilt tailwind.min.css, so no rebuild.

Reproduced + verified headless: in the filter-bar/menu overlap region the
topmost element flips from the filter bar's <select> to the instrument menu.
(The tuner panel #tuner-plugin-ui is z-[1000] on document.body and was already
on top — unaffected.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
(cherry picked from commit 45bfd9cc9c6981a10a9f2441638b7e5bee760dc8)

static/v3/shell.js

@@ -145,7 +145,12 @@
     function renderTopbar() {
         const bar = document.getElementById('v3-topbar');
         if (!bar) return;
-        bar.className = 'sticky top-0 z-20 bg-fb-sidebar/80 backdrop-blur';
+        // z-30 (not z-20): the global topbar must stack above per-screen sticky
+        // bars — notably the v3 Songs filter/search bar, which is also
+        // `sticky top-0 z-20 backdrop-blur` and, being later in source order,
+        // otherwise wins the z-index tie and paints over the topbar's instrument
+        // selector / tuner dropdowns (slopsmith#857 regression).
+        bar.className = 'sticky top-0 z-30 bg-fb-sidebar/80 backdrop-blur';
         bar.innerHTML =
             // Row 1 — top utility bar: search.
             '<div class="flex items-center gap-4 px-4 md:px-8 pt-4">' +