Investigate OFX support for broader bank compatibility

[corentin-core]

Context

The app currently supports BNP Paribas (proprietary Excel export) and Swile (ZIP/CSV). OFX (Open Financial Exchange) is a widely supported standard for bank data exchange — most French banks (Crédit Agricole, Société Générale, Boursorama, etc.) can export in this format. Adding OFX support could make the app broadly compatible without writing per-bank adapters.

Investigation needed

• Verify which French banks offer OFX export (BNP included?)
• Evaluate ofxparse library (data fields available, maintenance status)
• Assess what's lost vs proprietary exports (e.g. BNP provides categories, OFX typically doesn't)
• Determine if the existing Categorizer can compensate for missing categories
• Check if OFX provides additional useful fields (transaction type, check number, etc.)

Proposed solution (if investigation is positive)

• Add a generic OFXBankAdapter implementing BankAdapterInterface
• Rely on Categorizer for category assignment (no bank-provided categories)
• Keep existing BNP/Swile adapters for users who prefer richer data

Acceptance criteria

• Investigation results documented in the issue
• Decision: implement or not (with rationale)
• If yes: design for OFXBankAdapter proposed before implementation