perf(mail): cut unnecessary render/listener work in mobile nav

[real-venus]

Summary

Scoped performance pass on the existing Mobile Navigation surface (issue #978). Reduces unnecessary global listeners and per-keystroke re-renders. No behavior, copy, or visual change — strictly fewer wasted renders/listeners.

Closes #978

Hotspots identified (before changing code)

• Topbar attached a global scroll listener in capture phase that stayed mounted at all times, so it fired on every scroll anywhere in the app (email list, reader panes) even when no dropdown was open and there was nothing to reposition.
• Sidebar held the new-folder input text in its own state, so typing a folder name re-rendered the entire sidebar nav — every section item and every FolderButton (each a motion.button) — on every keystroke.

Changes

• Topbar — gate global listeners — scroll/resize now subscribe only while a dropdown is open (if (!anyOpen) return). In the common all-closed state, no global listener runs. Reposition-on-open and reposition-on-scroll-while-open behavior is unchanged.
• Sidebar — isolate add-folder input — extracted AddFolderInput, which owns its own text state. Typing is now scoped to the input; the folder list no longer re-renders per keystroke. Markup, animation, Enter/Escape/cancel behavior all preserved.

Performance note (reasoned)

• Topbar: eliminates a capture-phase scroll handler during normal scrolling whenever no dropdown is open (i.e. almost always).
• Sidebar: per-keystroke work in the add-folder flow drops from "re-render the whole sidebar subtree" to "re-render one input".

Scope

2 files, both within the issue's listed paths. No new tool/feature, no real data, no broad refactor.