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_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