[FEATURE] Desktop app: jump directly to a session/tab by index (Cmd+1..9)

Summary

Add keyboard shortcuts to jump directly to a specific session/tab by index in the Claude Code desktop app, similar to browser tab shortcuts (Cmd+1..Cmd+9 on macOS, Ctrl+1..Ctrl+9 on Windows/Linux).

Current behavior

The desktop app only supports relative session navigation — Ctrl+Tab / Ctrl+Shift+Tab and Cmd+Shift+] / Cmd+Shift+[ cycle to the next/previous session. With several sessions open, reaching a specific one means cycling through everything in between.

Requested behavior

Direct index-based shortcuts: Cmd+1 focuses the first session, Cmd+2 the second, etc. (Cmd+9 could jump to the last, as browsers do). This matches the muscle memory most users already have from browsers and editors.

Nice to have

Allowing these to be remapped (e.g. to Option+1..9) — the desktop app currently exposes no keybinding customization; ~/.claude/keybindings.json only affects the CLI/TUI.

Environment

Claude Code desktop app (macOS).