feat: Soroban contract authorization tree visualization (#850)

[legend-esc]

Add nested authorization entry visualization to the Soroban invocation UI so developers can verify required signers and their authorized invocation trees before signing.

Changes:

• stellar.ts: extend ContractSimulationResult with authEntries field; add serializeAuthEntry/serializeAuthInvocation helpers that convert XDR SorobanAuthorizationEntry values to plain JSON-safe objects
• AuthorizationTree.tsx: new component rendering the auth entry tree — credentials badge (source account vs address), nonce/expiry ledger for address credentials, collapsible invocation nodes with sub-invocation counts, security notice, ARIA tree roles
• ContractInteraction.tsx: import and render AuthorizationTree after simulation result; falls back silently when authEntries is empty
• tests/unit/components/AuthorizationTree.test.tsx: 21 tests covering primary flow, boundary cases (empty, nested, multi-signer, unknown function types), and failure cases (null/undefined/non-array input, malformed entries)
• docs/features/soroban-auth-tree.md: feature guide with usage, security notes, compatibility table, data model reference, and component API
• README.md: add feature guide entry

Closes #850

How was this tested?

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[Manuelshub]

@legend-esc Please resolve conflicts.

[legend-esc]

@legend-esc Please resolve conflicts.

Done