Add "S" keyboard shortcut to toggle activity sidebar

[saptadeep12]

Summary

Adds an "S" keyboard shortcut to toggle the activity sidebar, so power users don't have to click the navbar.

Fixes #527

Changes

• Added keydown listener in client/src/pages/Dashboard.jsx (useEffect) that toggles showActivity on S/s
• Ignores shortcut when typing in an input/textarea
• Cleans up listener on unmount
• Updated shortcuts help modal: S — Toggle activity

Testing

• S toggles sidebar correctly
• No trigger while typing in inputs
• Navbar click still works (no regression)