Implement Ledger transaction signing

[Gojv-byte]

Summary
This PR adds end-to-end Ledger hardware wallet signing support to the dashboard’s transaction flow. It enables users to connect a Ledger device, reuse the active Ledger session, sign Stellar transaction XDR, and return a valid signed envelope for submission to the network.

What Changed
Added Ledger connection/session management with WebUSB/WebHID detection and Stellar app initialization
Implemented transaction signing for raw Stellar transaction objects and unsigned XDR envelopes
Validated network passphrase, derivation path, public key, and signature output before assembling the signed transaction
Added clear user-facing error handling for unsupported browsers, rejection, locked devices, and app state issues
Integrated the flow into the existing transaction signer UI and wallet state
Added test coverage for successful signing and rejection scenarios
Why
The app previously lacked a reliable Ledger signing path. This creates a gap for users who rely on hardware wallet security and prevents Ledger-based signing from being used in the transaction workflow.

Validation
Executed:
npx vitest run src/lib/wallet/tests/ledger.test.js

Result:

1 test file passed
3/3 tests passed

closes: #759

Impact
This closes the core Ledger signing gap in the wallet flow and gives users a secure, hardware-backed transaction signing path within the dashboard.

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[Manuelshub]

@Gojv-byte Please resolve the conflict