Sidebar toggle shortcuts split into left / right

What changed

The single "Toggle Sidebar" and "Toggle Topics" shortcuts are replaced by "Toggle Left Sidebar" (Cmd/Ctrl+[) and "Toggle Right Sidebar" (Cmd/Ctrl+]), now listed under the Topic group in Settings → Shortcuts. The default key bindings are unchanged.

Why this matters to the user

In Settings → Shortcuts the entries are relabeled to "Toggle Left Sidebar" / "Toggle Right Sidebar". The right-side shortcut (Cmd/Ctrl+]) now toggles the v2 right-side pane instead of the old topics list. Users who customized the old shortcuts keep their bindings: v1 toggle_show_assistants / toggle_sidebar migrate to the left shortcut, toggle_show_topics migrates to the right shortcut.

What the user should do

Nothing — automatic. Bindings are preserved through migration.

Notes for release manager

• Bindings are unchanged from v1; this is essentially a relabel plus the right shortcut now targeting the v2 right pane.
• Do NOT describe any "topic sidebar auto-opens" change — that behavior was lost earlier in the chat-page UI refactor (the MainSidebar listener for SHOW_TOPIC_SIDEBAR was removed) and is unrelated to this shortcut work.
• introduced_in_pr is TBD — fill with the feat/chat-page PR number (or commit hash) when committed.