Add offline Cashu V3 decoding for Mission 84

[Nitjsefnie]

Summary

Adds a hands-on Mission 84 exercise that decodes a bundled Cashu V3 sample entirely in the browser and reports its mint URL, amount, and proof count. The token remains local; malformed or unsupported input is rejected before the mission can complete.

Related Issues and Pull Requests

Relates to #84

This implements the issue's self-contained Mission 84 path. The umbrella issue's other proposed hands-on missions remain open.

Changes

• Added lesson copy and an official bundled V3 sample for the offline decoding exercise.
• Added base64url, UTF-8, and JSON decoding with validation for proof amount, V1 keyset ID, secret, and compressed secp256k1 point metadata.
• Added canonical completion facts and server-side checks without submitting the raw token.
• Added browser and API coverage for valid, malformed, unsupported, and later-proof inputs.

Testing

• Exact-candidate run 32870646593 tested c0fb6d6e72827d4d6d8773118d5a735957c39c9e; all 66 focused browser assertions passed after a locked frontend install, backend release build, Vite launch, and Chrome execution.
• Test-first run 32870302111 failed the five expected zero-completion assertions before implementation.
• Isolated mutations for generic keyset IDs, empty secrets, and generic point hex were rejected by runs 32870986920, 32870990374, and 32870993369.

Bugs Discovered

• badge-share.test.mjs seeds an incomplete Money Basics tree #100 badge-share.test.mjs seeds an incomplete Money Basics tree
• cargo audit reports 11 vulnerabilities in locked Nostr dependencies #101 cargo audit reports 11 vulnerabilities in locked Nostr dependencies

Follow-ups / Known Limitations

• This exercise validates supported V3 metadata offline; it does not contact the mint or verify proof signatures against mint keys.
• V4/CBOR cashuB tokens are not decoded by this exercise and receive a focused explanation.

Dependencies

• Promoted @noble/curves to a direct production dependency at the already locked version 1.9.7; its resolved version and integrity entry are unchanged.

Generated by GPT-5.6 Sol (brief, implementation, testing, review, verification)

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