refactor: components read the obsidian surface from its token, not a typed rgba (3108)

Thirteen rgba(20, 23, 26, ...) across six components (the task counted
eleven). They now use --v-theme-background, which Vuetify maps to obsidian;
ArtistHeader's banner fade uses --fc-chrome-rgb, since it is nav-style
chrome. A spec fails if the literal reappears outside a comment. Values are
unchanged, so nothing should render differently.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/test/paletteLiterals.spec.js

@@ -0,0 +1,40 @@
+// The obsidian surface written out as a number instead of read from its token.
+//
+// fabled-tokens.js is the palette's single copy ("do not hand-edit hexes;
+// re-mirror here"). Thirteen hand-typed `rgba(20, 23, 26, …)` across six
+// components meant a changed obsidian would miss most of the UI (#3108).
+// Components read `--v-theme-background` (Vuetify maps background → obsidian)
+// or, for nav-style chrome, `--fc-chrome-rgb`. Comments may still name it.
+
+import { readdirSync, readFileSync, statSync } from 'node:fs'
+import { join, relative } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+import { describe, expect, it } from 'vitest'
+
+const SRC = fileURLToPath(new URL('../src', import.meta.url))
+const LITERAL = /rgba?\(\s*20\s*,\s*23\s*,\s*26\b/
+
+function vueFiles (dir) {
+  return readdirSync(dir).flatMap((name) => {
+    const path = join(dir, name)
+    if (statSync(path).isDirectory()) return vueFiles(path)
+    return name.endsWith('.vue') ? [path] : []
+  })
+}
+
+function stripComments (text) {
+  return text.replace(/\/\*[\s\S]*?\*\//g, '').replace(/<!--[\s\S]*?-->/g, '')
+}
+
+describe('palette literals', () => {
+  it('no component hand-types the obsidian surface', () => {
+    const offenders = vueFiles(SRC).flatMap((path) =>
+      stripComments(readFileSync(path, 'utf8'))
+        .split('\n')
+        .filter((line) => LITERAL.test(line))
+        .map((line) => `${relative(SRC, path)}: ${line.trim()}`)
+    )
+    expect(offenders).toEqual([])
+  })
+})