branding: add EverShelf logo to kiosk splash + web header/preloader

- assets/img/logo/logo.png: trimmed full logo (icon + text, transparent bg)
- assets/img/logo/logo_icon.png: icon-only crop (no text, for header)
- drawable-*/ic_logo.png: multi-density PNGs for Android splash (mdpi→xxxhdpi)
- activity_kiosk.xml: replace ic_launcher_foreground with ic_logo at 260dp,
  remove redundant 'EverShelf' text row (already in logo image)
- index.html: add logo_icon.png in header title, logo.png in preloader
- style.css: add .app-preloader-logo and .header-logo-icon rules

assets/css/style.css

@@ -103,6 +103,21 @@ body {
     font-weight: 600;
     letter-spacing: 0.5px;
 }
+.app-preloader-logo {
+    height: 120px;
+    width: auto;
+    object-fit: contain;
+    filter: drop-shadow(0 2px 8px rgba(0,0,0,0.4));
+}
+.header-logo-icon {
+    height: 28px;
+    width: auto;
+    object-fit: contain;
+    display: inline-block;
+    vertical-align: middle;
+    margin-right: 4px;
+    filter: drop-shadow(0 1px 3px rgba(0,0,0,0.3));
+}
 
 .header-content {
     display: flex;