fix: Update zoom button to preferred purple shade #5c59b6

Changed zoom button color from #9b59b6 to #5c59b6 for better visual appeal.
The new shade is more blue-tinted, creating a vibrant indigo/periwinkle appearance
that remains distinct from the info button blue.

Changes:
- Updated zoom-toggle-btn background: rgba(92, 89, 182, 0.7)
- Updated hover state: #5c59b6
- Updated at-bottom state: #5c59b6
- Updated test output to reflect correct hex code

Test verified: All buttons visible and colors distinct at all viewports.

tests/mjs/67-button-colors-and-visibility-test.mjs

@@ -260,7 +260,7 @@ async function testViewport(browser, viewport) {
     if (failures.length === 0) {
         console.log('✅ ALL TESTS PASSED\n');
         console.log('Button colors are distinct:');
-        console.log('  🔍 Zoom button: Purple (#9b59b6)');
+        console.log('  🔍 Zoom button: Purple (#5c59b6)');
         console.log('  ℹ️  Info button: Blue (#3498db)');
         console.log('\nButton visibility works correctly:');
         console.log('  ✅ All buttons visible at desktop viewport (>900px)');