[FEATURE]: Shareable link + QR code for each invoice

Feature and its Use Cases

Add a generated shareable public link and QR code for every invoice. The link and QR should point to a read-only invoice view page that can be opened without requiring wallet login (or with controlled access, depending on privacy settings).

• Generate a shareable URL and QR code that contain encoded invoice data directly (instead of only linking by invoice ID).

• Instant invoice rendering after opening/scanning, reduced dependency on backend fetch, and easier offline/portable sharing.

• Quick invoice sharing in chat/email, scan-to-view in physical invoices, and environments with limited connectivity.

Additional Context

• Prefer compressed + signed payload to prevent tampering.
• Add fallback when payload exceeds practical QR/URL size limits.

Code of Conduct

• I have joined the Discord server and will post updates there
• I have searched existing issues to avoid duplicates