feat(SorobanScreen): validate contract IDs and manage saved contracts (#347)

[BernardOnuh]

Description

Enhances the existing Soroban screen saved-contracts / Stellar Expert implementation for #347 with stricter, more convenient contract-ID handling.

1. Contract ID validation

• Only a well-formed Soroban contract ID (/^C[A-Z0-9]{55}$/, the C-prefixed strkey format) is treated as valid.
• The last-used contract is pre-filled on mount only when valid; malformed history entries are skipped instead of producing a broken active input or fake links.
• The Stellar Expert link no longer appears for empty or invalid non-empty IDs — it only shows for a genuine contract.

2. Saved Contracts management

• The panel now shows as soon as one saved contract exists (previously required >1).
• Each saved contract gets a × remove button (removes it from the dropdown and localStorage), plus a "Clear all" action.
• Full contract ID is exposed on hover via title for quick identification.
• Replaces the useEffect-in-setState sync (fixes a react-hooks/set-state-in-effect lint error) with a clean per-render read + removed set.

Acceptance Criteria

• Last-used (valid) contract ID pre-filled on mount
• Saved Contracts dropdown lists and applies saved IDs; now also removable / clearable
• Stellar Expert link appears for valid non-empty contract IDs
• npm run build passes

Tests

src/screens/SorobanScreen.test.tsx now 18 tests, all passing. Added coverage for:

• pre-fill skipping invalid first entries
• listed/applied + remove + clear-all behavior
• full-ID hover title
• no Stellar Expert link for invalid-but-non-empty IDs

Screenshot: The Saved Contracts dropdown (with remove buttons & Clear all) and the Stellar Expert link appear in the Soroban screen.