feat: add Groth16 zk-proof verifier for anonymous meter readings (#64)

[real-venus]

Zero-Knowledge Proof Verifier for Anonymous Meter Reading Submissions (#64)

Closes #64

What's included

File	Responsibility
src/types/zk.ts	Shared types: private/public circuit signals, proof, verification key, submission state, worker messages, and range/freshness bounds.
src/services/keyCache.ts	IndexedDB cache for the ~150 MB proving key — resumable downloads via HTTP Range requests, SHA-256 integrity check, and LRU eviction.
src/workers/zkProver.worker.ts	Web Worker running the snarkjs Groth16 prover off the main thread; lazily loads snarkjs (local module → pinned ESM CDN). Returns { proof, publicSignals }.
src/services/zkProver.ts	Orchestrator. prepareInputs() validates the reading and builds the witness (field elements → decimal strings, random 248-bit salt); generateProof() caches the key, drives the worker (zkey transferred, not copied), returns a proof hash.
src/services/zkVerifier.ts	Fetches the verification key from the Soroban zk_verification_key data entry, caches it 24h, runs groth16.verify.
src/hooks/useZKSubmission.ts	React hook exposing the prove → submit → verify pipeline with weighted progress (0–100), status, proofHash, plus cancel/reset.
src/components/panels/ZKSubmissionPanel.tsx	Stepper UI (cache key → prove → submit → verify) with progress bar, input validation, cancel, and proof-hash display.
public/wasm/zk/	Artifact placeholders + README (real circuit_final.zkey is CDN-hosted; configured via NEXT_PUBLIC_ZK_ZKEY_URL / NEXT_PUBLIC_ZK_ZKEY_SHA256).
tests/unit/zkProver.test.ts, tests/unit/keyCache.test.ts	Input validation/range bounds, salt generation, hex→field conversion, SHA-256 integrity vectors.