Add sidebar context menu (rename, export, delete)

[phillsatellite]

Summary

• Right-click or tap+hold (600ms) on any conversation in the sidebar opens a context menu
• Menu options: Edit name (inline rename with Enter/Escape/blur), Export (loads conversation + opens export modal), Delete (removes from Firestore, clears output if active)
• Menu position is clamped to viewport so it never clips off-screen
• All three actions wired to Firestore handlers in App.jsx

Test plan

• Right-click a conversation → context menu appears near cursor
• Tap+hold on mobile → context menu appears
• Edit name → type new name → Enter commits, Escape cancels, clicking away commits
• Export → opens export modal with that conversation's text
• Delete → conversation removed from sidebar; active conversation cleared if deleted