feat: add language selector with i18n support

- Add createLanguageSelector function to amipro_utils.js
- Add language selector CSS styles to modern.css and core.css
- Support EN/JA language switching with localStorage persistence

files/modern.css

@@ -400,3 +400,55 @@ body {
     left: -38px;
   }
 }
+
+.lang-selector-wrapper {
+  margin-left: 12px;
+  background: var(--bg-panel);
+  border: 1px solid var(--border-soft);
+  border-radius: 8px;
+  padding: 4px;
+  backdrop-filter: blur(16px);
+  display: inline-block;
+}
+
+.lang-selector-floating {
+  position: fixed;
+  top: 16px;
+  right: 16px;
+  z-index: 9999;
+  margin-left: 0;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+  transition: box-shadow 0.2s ease;
+  display: inline-block;
+  width: fit-content;
+  max-width: fit-content;
+  flex-shrink: 0;
+  white-space: nowrap;
+}
+
+.lang-selector-floating:hover {
+  box-shadow: 0 6px 20px rgba(0, 0, 0, 0.2);
+}
+
+.lang-selector {
+  background: transparent;
+  border: 1px solid var(--border-soft);
+  border-radius: 8px;
+  color: var(--text-bright);
+  padding: 4px 8px;
+  font-size: 0.85rem;
+  font-weight: 600;
+  cursor: pointer;
+  outline: none;
+  appearance: none;
+  -webkit-appearance: none;
+}
+
+.lang-selector option {
+  background: #1e293b;
+  color: var(--text-bright);
+}
+
+.lang-selector:hover {
+  border-color: var(--brand-primary);
+}