feat(frontend): add guardian secret sharing recovery for credentials

[dotmantissa]

What does this PR do?

This PR introduces client-side Guardian Secret Sharing Recovery for credentials stored in browser localStorage. A holder can now split their 256-bit AES encryption key into N Shamir secret shares across trusted guardians or backup devices, requiring a threshold of K shares (e.g. 2-of-3) to reconstruct the key and restore their credentials. Guardians only receive key shares with zero credential data, ensuring privacy while offering a robust recovery option if browser storage is wiped.

Closes #431

Type of change

• Bug fix
• New feature / credential type
• Refactor / cleanup
• Docs
• CI / tooling

Merge requirements

• CI is green — cargo test (contracts), pnpm tsc --noEmit (frontend), pnpm build (frontend), circuit tests — all green
• Greptile confidence ≥ 4/5 — all review comments addressed, no unresolved threads
• Circuit changes: fixtures/<type>/ artifacts updated
• No NEXT_PUBLIC_ prefix on server-only env vars
• No identity fields stored or logged after KYC provider call
• prehash:false preserved on any issuer signing path touched
• Issuer private key never referenced from client-bundled code

✅ Merge requirements

• All CI checks pass (contracts / frontend / circuits as applicable)
• Every Greptile review comment is addressed — no unresolved review threads
• Greptile confidence score is 4/5 or higher

Notes for reviewers

• Implemented pure Galois Field GF(256) arithmetic and Shamir's Secret Sharing in lib/shamir.ts.
• Encrypted backup creation, share serialization/parsing (both JSON and compact SC-SHARE:... armored string), key fingerprint verification, and decryption logic are encapsulated in lib/guardian.ts.
• Built GuardianRecoveryModal.tsx containing both the Key Splitting setup flow and the guided share-collection recovery flow.
• Dynamically imported the modal in app/holder/page.tsx to preserve the 16 kB bundle limit for the holder route.
• Added comprehensive unit test suites in lib/__tests__/shamir.test.ts and lib/__tests__/guardian.test.ts covering mathematical properties, threshold variations, edge cases, tamper resistance, and malformed inputs.

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[Psalmuel01]

The guardian recovery via Shamir secret sharing (#431) is well-designed: K-of-N threshold, guardians receive only key shares (never credential data), client-side reconstruction. It just went conflicting though, #490 (perf telemetry) merged into holder/page.tsx right after this. Please rebase onto latest main. Note #441 also restructured the holder page into components/holder and lib/hooks, so fold the Guardian recovery entry point into that new structure rather than the old monolithic page.

[dotmantissa]

The guardian recovery via Shamir secret sharing (#431) is well-designed: K-of-N threshold, guardians receive only key shares (never credential data), client-side reconstruction. It just went conflicting though, #490 (perf telemetry) merged into holder/page.tsx right after this. Please rebase onto latest main. Note #441 also restructured the holder page into components/holder and lib/hooks, so fold the Guardian recovery entry point into that new structure rather than the old monolithic page.

@Psalmuel01
Implemented the fix, kindly review, and merge.