Add comprehensive test coverage (frontend + Rust)

[joshpainter]

Summary

• Adds ~268 new tests across frontend and Rust, covering areas that previously had zero test coverage
• Sets up Vitest infrastructure with Tauri IPC mocking for the frontend (previously no frontend tests existed)
• Expands Rust test coverage to sage-keychain, sage-database, sage-config, and sage parse utilities
• Adds lint + frontend test steps to CI workflow

Frontend (170 tests, 7 files)

• Test infrastructure: Vitest + jsdom + Testing Library, with global mocks for all Tauri APIs and plugins
• Pure functions: amount conversion (toMojos/fromMojos), address encoding/validation, hex utilities, URL validation, deepMerge
• WalletConnect: Zod schema validation for all 17 commands, handler tests for CHIP-0002, offers, and high-level commands
• State: Zustand store tests (wallet, offer, navigation) with mocked Tauri bindings

Rust (98 tests, 8 files)

• sage-keychain (16 tests): encrypt/decrypt round-trips, wrong password, tampered data, keychain CRUD, serialization
• sage-database (32 tests): blocks, offers, collections, mempool items, type conversion utils (in-memory SQLite)
• sage-config (23 tests): config defaults/TOML round-trip, network inheritance/lookup, v1→v2 migration
• sage parse (27 tests): parse_asset_id, parse_coin_id, parse_hash, parse_amount, parse_signature_message, and more

CI

• Adds pnpm lint and pnpm test steps to the build workflow

Test plan

• pnpm test — 170 frontend tests pass
• cargo test -p sage-keychain — 16 tests pass
• cargo test -p sage-database — 32 tests pass
• cargo test -p sage-config — 26 tests pass (3 pre-existing + 23 new)
• cargo test -p sage -- utils::parse — 24 tests pass
• CI workflow passes with new lint + test steps

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