merge #254: mobile explain unavailable sidebar and prevent overflow

# Conflicts:
#	README.md
#	README_EN.md
#	dsh.plugin.json
#	package.json
#	src/client/service.ts

README.md

@@ -425,7 +425,7 @@ pnpm watch        # tsdown --watch
 - Office 三件套预览（.docx/.xlsx/.pptx）已移至「推荐插件」（Office 预览插件，见设置页「添加插件」弹窗）；未安装时此类文件走代码/下载查看兜底
 - 浏览器沙箱无登录态/第三方 Cookie 受限，部分站点登录需走弹窗；被 `X-Frame-Options`/`frame-ancestors` 拒绝嵌入的站点（如 arxiv.org）显示原因面板（含「在浏览器中打开」）；iframe 内部跳转不进后退栈
 - HTML 预览渲染的是已保存文件（不反映未保存草稿）
-- 移动端（<768px）无底部面板：进入窄屏时其标签页一次性并入右侧栏（迁移后回桌面仍保留在右侧栏），桌面端的底部面板只在宽视口下可用；移动端底部首展自动开终端不触发
+- 移动端（<768px）无底部面板：进入窄屏时其标签页一次性并入右侧栏（迁移后回桌面仍保留在右侧栏），桌面端的底部面板只在宽视口下可用；移动端底部首展自动开终端不触发。未选中会话时，点按弱化开关会显示选择会话提示；选中会话后开关打开全宽抽屉
 
 ## 🖥️ 平台支持
 

README_EN.md

@@ -422,7 +422,7 @@ pnpm watch        # tsdown --watch
 - Office-suite preview (.docx/.xlsx/.pptx) moved to the recommended office plugin (see the "Add plugins" modals in settings); without it these files fall through to the code/download fallbacks
 - Browser sandbox has no login state / third-party cookies are restricted; some sites need popup login; sites that refuse embedding via `X-Frame-Options`/`frame-ancestors` (e.g. arxiv.org) show a reason panel (with "Open in browser"); in-iframe navigation does not enter the back stack
 - HTML preview renders the saved file (not unsaved drafts)
-- No bottom panel on mobile (<768px): on narrow screens its tabs merge into the right sidebar once (after migrating back to desktop they stay in the right sidebar); the desktop bottom panel is only available on wide viewports; auto-open terminal on first bottom-panel expand does not trigger on mobile
+- No bottom panel on mobile (<768px): on narrow screens its tabs merge into the right sidebar once (after migrating back to desktop they stay in the right sidebar); the desktop bottom panel is only available on wide viewports; auto-open terminal on first bottom-panel expand does not trigger on mobile. Without a selected session, tapping the subdued toggle shows the select-session message; with a selected session, it opens the full-width drawer
 
 ## 🖥️ Platform Support
 

src/client/Sidebar.tsx

@@ -1030,18 +1030,20 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
   }, [ctx, sessionId, cwd])
 
   if (state === undefined || sessionId === undefined) {
+    // Keep the unavailable controls focusable: touch users have no hover, so
+    // focus is the only way the existing Tooltip can explain what is missing.
     return (
       <div data-dsh-panel-host {...osFileDragShield}>
         <div className={css.toggleCluster} data-dsh-toggle-cluster>
           {!narrow && (
             <Tooltip label={t('noSession')} side="bottom" delayMs={500}>
-              <button type="button" className={css.toggleButton} disabled aria-label={t('noSession')}>
+              <button type="button" className={css.toggleButton} aria-disabled="true" aria-label={t('noSession')}>
                 <IconPanelBottomOutline16 />
               </button>
             </Tooltip>
           )}
           <Tooltip label={t('noSession')} side="bottom" delayMs={500}>
-            <button type="button" className={css.toggleButton} disabled aria-label={t('noSession')}>
+            <button type="button" className={css.toggleButton} aria-disabled="true" aria-label={t('noSession')}>
               <IconPanelRightOutline16 />
             </button>
           </Tooltip>

src/client/sidebar.module.css

@@ -105,17 +105,19 @@
     color var(--ds-transition-duration-slow) var(--ds-ease-in-out);
 }
 
-.toggleButton:hover:not(:disabled) {
+.toggleButton:hover:not(:disabled):not([aria-disabled='true']) {
   background: var(--dsw-alias-interactive-bg-hover);
   color: var(--dsw-alias-label-primary);
 }
 
-.toggleButton:disabled {
+.toggleButton:disabled,
+.toggleButton[aria-disabled='true'] {
   opacity: 0.4;
   cursor: default;
 }
 
 .panel {
+  box-sizing: border-box;
   position: absolute;
   top: 0;
   right: 0;