Add export and share functionality

[jglopez]

Allow users to share or save a snapshot of the bill split.

Output formats to consider: PNG, PDF, CSV

Sharing:

• Use the Web Share API where available to trigger the OS-level share sheet (works on iOS and Android without any native code)
• Fall back to a copy-to-clipboard button on desktop

Formatting:

• Output should be mobile-friendly — most recipients will view it on their phone
• The current `SummarySection` component is already isolated and a good candidate for the export view