Add keyboard shortcuts panel to frontend-v3

[luaroncrew]

Summary

• Adds a fixed ? button (bottom-right corner) that toggles a keyboard shortcuts modal
• Modal slides in with a fade/translate animation and lists all available shortcuts
• No external libraries — pure HTML/CSS/JS

Shortcuts added

Key	Action
?	Toggle shortcuts panel
Esc	Close modal → close image zoom → back to selection (priority order)
R	Refresh chart list
← / →	Navigate charts with accent-outline highlight, wraps around
Enter	Open focused chart

Test plan

• Open frontend-v3/index.html in browser (with backend running)
• Click ? button → modal appears
• Press ? key → modal toggles
• Press Esc → modal closes; press again → goes back to selection
• Press R on selection view → charts reload
• Press ←/→ → cards get purple outline, scroll into view
• Press Enter on focused card → opens prediction