Desktop app: add split panes and numbered tab shortcuts (Cmd-1/2/3) for power users

Coming from Warp, the new Claude Code desktop app feels like a step back for keyboard-first workflows. Two things would make it a homerun:

1. Split panes — ability to view 2+ chats side-by-side in one window. Currently sessions are one-at-a-time in the sidebar.
2. Numbered session shortcuts — Cmd-1, Cmd-2, Cmd-3 etc. to jump directly to a specific session, like every modern terminal (Warp, iTerm, Ghostty). Ctrl+Tab cycling works but is slower than direct-access.

Bonus: a command palette (Cmd-K) for session switching by name would close the gap completely.

Right now I'm sticking with Warp + CLI because the muscle memory is faster. Would love to switch to the desktop app once parallel session management catches up.