style(LocalLibrary): homogenize filter dropdown and artist alpha-index

- Reduce filter panel text sizes from 13px to 12px for consistency
- Update artist alpha-index to use same styling as albums/tracks tabs
- Replace boxed buttons with clean text-only alpha letters

static/filter.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?><!-- Uploaded to: SVG Repo, www.svgrepo.com, Generator: SVG Repo Mixer Tools -->
+<svg width="800px" height="800px" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M4.22657 2C2.50087 2 1.58526 4.03892 2.73175 5.32873L8.99972 12.3802V19C8.99972 19.3788 9.21373 19.725 9.55251 19.8944L13.5525 21.8944C13.8625 22.0494 14.2306 22.0329 14.5255 21.8507C14.8203 21.6684 14.9997 21.3466 14.9997 21V12.3802L21.2677 5.32873C22.4142 4.03893 21.4986 2 19.7729 2H4.22657Z" fill="#000000"/>
+</svg>