Title: [Feature Request] Add Offline-First Mode + USSD Fallback for Low-Bandwidth Regions

Problem DocuSeal is great for modern web signing, but it assumes users have stable internet and smartphone access. For users in Ghana and similar markets:

1. Internet is intermittent: Field agents, market traders, or construction foremen often lose connection mid-signing. Current flow fails with no auto-recovery.
2. Feature phones are still 40%+ of devices: USSD is the primary channel for many SMEs, but DocuSeal requires a browser.
3. Data costs: Loading full PDF + WYSIWYG editor on 3G consumes 5-10MB per document. Too expensive for bulk signing.

This limits DocuSeal adoption for SMEs in Africa and other emerging markets.