fix: fall back to home dir when terminal cwd does not exist

[whitesockcat]

Summary

• Terminal tile crash fix: When a terminal tile remembers a remote/WSL cwd (e.g. /home/user/) that does not exist on the local macOS host, the spawned shell exits immediately, causing the tile to flash and disappear. Added fs.accessSync validation before spawning — falls back to os.homedir() when the path is inaccessible.
• Sidebar shortcut change: Changed the sidebar-files toggle from Cmd+B to Cmd+Shift+B to avoid conflicts with common terminal keybindings (e.g. tmux prefix). Updated menu accelerator and tooltip to match. Cmd+\ remains as an alternative.

Test plan

• Create two terminal tiles, one with a remote/WSL target so its cwd is a non-local path (e.g. /home/user/)
• Close and reopen the app so the cwd is remembered
• Create a third terminal tile — it should open in the home directory instead of crashing
• Verify Cmd+Shift+B toggles the sidebar
• Verify Cmd+\ still toggles the sidebar

🤖 Generated with Claude Code

[yiliush]

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[whitesockcat]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.