feat: build, inspect, and export fee-bump transactions with inner-env…

[praizeD10]

closes #758

…elope details

Three things were broken for fee-bump transactions:

1. Builder.tsx uses stellar.ts buildTransaction/simulateTransaction, which had no fee-bump support at all. A feeBump operation would silently skip during build and crash on simulate with a misleading error.

2. TransactionSigner accepted any XDR but never inspected the envelope — pasting a FeeBumpTransaction gave no way to see what you were actually signing (inner source, operations, fee, signature count were all invisible).

3. No utility existed to parse XDR and safely identify its type or surface inner-envelope details without throwing.

Changes:

src/utils/feeBumpInspector.ts (new)
Pure utility with no side effects. inspectEnvelope(xdr, network) returns a
typed discriminated union. For FeeBumpTransaction: outer hash, feeSource,
outer fee, and full inner envelope (hash, source, fee, sequence, signatures,
operation list). For plain Transaction: the same fields directly. Never
throws — callers check .ok before accessing .envelope.

src/components/dashboard/TransactionSigner.tsx

• Added envelopeInfo state driven by inspectEnvelope on every XDR change.
• handleXdrChange now also clears signedXdr and error on new input so stale state never lingers alongside a new paste.
• EnvelopeDetails component renders below the textarea: for fee-bump it shows a nested two-panel layout (outer wrapper + inner transaction); for plain transactions it shows the standard field set.

src/lib/stellar.ts

• buildTransaction return type widened to Transaction | FeeBumpTransaction. Fee-bump path added before the account-load path: validates feeSource, baseFee, innerTransaction XDR with explicit error messages, then calls TransactionBuilder.buildFeeBumpTransaction. Mixed fee-bump + other ops rejected with a clear error.
• simulateTransaction: skips validateSimulationParams (which requires a sourceAccount) when the only operation is feeBump; uses instanceof FeeBumpTransaction to compute operationCount correctly (inner ops + 1); guards the Soroban simulation path with the same check.
• exportTransactionXDR is unaffected — .toXDR() exists on both types.

tests/unit/lib/feeBumpInspector.test.js (new)
17 tests across 4 groups: primary fee-bump flow (type, feeSource, inner
details, hashes, fee ordering), plain transaction flow, boundary cases
(unsigned tx, whitespace input, unknown network), failure cases (empty
string, garbage, truncated XDR, null/non-string inputs — never throws).

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[Manuelshub]

@praizeD10 Please fix all the CI checks failure!

[Manuelshub]

@praizeD10 Please resolve conflicts so I can merge!!