feat(web): use static SVG asset for album fallback cover

Replaces the hand-rolled inline-data-URL placeholder with a static SVG
asset at /placeholders/album-fallback.svg. SvelteKit's adapter-static
publishes everything in web/static/ at the URL root and the Go binary
embeds the build output, so this resolves identically in dev and prod.

Operator can swap the artwork by replacing the file — no code changes.

Test updated to use toContain rather than strict equality, since jsdom
resolves relative URLs against the test origin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/static/placeholders/album-fallback.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="windows-1252"?>
+<!-- Uploaded to: SVG Repo, www.svgrepo.com, Generator: SVG Repo Mixer Tools -->
+<svg fill="#000000" height="800px" width="800px" version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 512 512" xml:space="preserve">
+<g>
+	<g>
+		<path d="M503.319,5.939c-5.506-4.705-12.783-6.767-19.958-5.635L169.555,49.852c-12.04,1.901-20.909,12.28-20.909,24.47v99.097    v156.903H99.097C44.455,330.323,0,371.073,0,421.161C0,471.25,44.455,512,99.097,512c54.642,0,99.097-40.75,99.097-90.839v-66.065    V194.588l264.258-41.725v136.169h-49.548c-54.642,0-99.097,40.75-99.097,90.839s44.455,90.839,99.097,90.839    S512,429.959,512,379.871v-66.065V123.871V24.774C512,17.529,508.827,10.646,503.319,5.939z"/>
+	</g>
+</g>
+</svg>