Add Excel/CSV import to ShareCost for migration from other apps#49

ShareCost would become much more useful if users could import existing shared-expense data from other apps or spreadsheets via Excel/CSV. Right now, for users who already manage trips, flat shares, or team expenses elsewhere, starting from zero creates friction and slows adoption. A native import feature would materially improve onboarding and reduce manual data entry.

I suggest adding an import workflow where users can upload an XLSX or CSV file and map columns such as group name, date, payer, participants, amount, currency, category, note, and settlement status. The system should include a preview screen, duplicate detection, error highlighting, and a downloadable sample template so users can validate the format before import.

This would be especially valuable for people moving from Wallet, Splitwise-style tools, or custom Excel trackers into ShareCost.

Great suggestion. Will add it into the web app. What do you think? There is already a wallet import feature, so it will nicely complement it.