Use theme-aware SVGs for download and offline actions

frontend/glass-controls.test.js

@@ -1,7 +1,7 @@
 const test=require('node:test'),assert=require('node:assert/strict');
 const {leadingGlyph}=require('./glass-controls.js');
 test('glass icon substitution preserves leading whitespace and remaining labels',()=>{
- assert.deepEqual(leadingGlyph('  ⬇ Save'),{kind:'save',original:'  ⬇',rest:' Save'});
+ assert.equal(leadingGlyph('Save'),null);
  assert.deepEqual(leadingGlyph('⚙️ Settings'),{kind:'settings',original:'⚙️',rest:' Settings'});
  assert.equal(leadingGlyph('A song with 🔍 in its title'),null);
  assert.equal(leadingGlyph('Clear all'),null);