fix: 文件选择器改为页内iframe弹窗 + 乱码编码修复 + API路径修复

- pickPath()从window.open改为页内iframe模态弹窗(遮罩+关闭按钮)
- files.html添加meta charset+CSS link,独立模式自动加载files.js
- API路径统一通过_apiUrl()构建,适配主面板和iframe两种上下文
- 独立iframe模式注入body/container覆写样式(无topbar)
- 选择模式栏移至底部margin-top:auto紧凑排列
- MD3风格toggle开关(显示隐藏复选框)
- CSS版本号缓存破坏

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

static/web_panel/css/style.css

@@ -453,8 +453,11 @@ h3{font-size:1rem;font-weight:500;letter-spacing:.15px}
 .fm-crumb-sep{color:var(--outline);font-size:.7rem;flex-shrink:0}
 .fm-resolved-crumbs{display:flex;align-items:center;gap:4px;font-size:.75rem;opacity:.8;flex-basis:100%}
 .fm-actions{display:flex;gap:6px;flex-shrink:0;align-items:center}
-.fm-toggle{display:flex;align-items:center;gap:4px;font-size:.75rem;color:var(--text-dim);cursor:pointer}
-.fm-toggle input{margin:0}
+.fm-toggle{display:flex;align-items:center;gap:8px;font-size:.8rem;font-weight:500;color:var(--text-dim);cursor:pointer;user-select:none}
+.fm-toggle input[type="checkbox"]{-webkit-appearance:none;appearance:none;width:36px;height:20px;background:var(--md-sys-color-surface-container-highest);border:2px solid var(--outline);border-radius:10px;cursor:pointer;position:relative;transition:.2s;margin:0;flex-shrink:0}
+.fm-toggle input[type="checkbox"]::after{content:"";position:absolute;top:2px;left:2px;width:12px;height:12px;background:var(--outline);border-radius:50%;transition:.2s}
+.fm-toggle input[type="checkbox"]:checked{background:var(--primary);border-color:var(--primary)}
+.fm-toggle input[type="checkbox"]:checked::after{left:18px;background:var(--on-primary)}
 .fm-list{flex:1;overflow-y:auto;scrollbar-width:thin;scrollbar-color:var(--outline) transparent}
 .fm-row{display:grid;grid-template-columns:32px 1fr 100px 140px;align-items:center;padding:10px 12px;cursor:pointer;border-radius:var(--shape-xs);transition:background .12s;gap:8px;font-size:.88rem}
 .fm-row:hover{background:rgba(208,188,255,.06)}