feat(wallet): add typed Stellar signing pipeline (#758)

[edehvictor]

Closes #758

Changes

• add a common Signer interface with production Freighter and session-reused Ledger WebHID implementations
• verify wallet network, perform Ledger health checks, enforce the device transaction-size limit, and classify extension/APDU/WebHID failures into actionable typed errors
• retry only Ledger timeouts using 2s, 4s, and 8s backoff while immediately surfacing locked-device, app-not-open, and user-rejection failures
• add deterministic signature aggregation that rejects mismatched transaction payloads
• expose a useSigner hook that detects available signers, respects the persisted wallet preference, and safely falls back when the preferred signer is unavailable
• add an accessible three-step signing progress component with inline recovery guidance
• add the official Freighter, Stellar SDK, and Ledger WebHID dependencies with a minimal lockfile update

Testing

• vitest run src/lib/signing/__tests__ src/hooks/__tests__/useSigner.test.tsx — 20 tests passed
• tsc -b --pretty false — signing code passes; the repository-wide run is currently blocked by pre-existing dev errors for missing recharts and missing Course fields in CreatorDetailPage.tsx
• targeted ESLint for all changed TypeScript/TSX files — passed
• git diff --check — passed

Notes

• Ledger behavior is exercised through deterministic error/retry tests; CI does not require a physical WebHID device.