feat(SorobanScreen): pre-fill last contract ID from localStorage, saved contracts picker, Stellar Expert link per contract

[k-deejah]

Problem

1. SorobanScreen has no persistent contract ID between sessions — closing the browser clears the last-used contract ID. With the localStorage history from issue b38 (contracts list), the most recently used contract ID should be pre-filled on mount.

2. SorobanScreen has no way to switch between multiple saved contracts — the localStorage history stores up to 5 contract IDs but the only way to use them is via the datalist suggestion in the input. A "Saved Contracts" sidebar panel or dropdown that directly sets the active contract ID would be more convenient.

3. SorobanScreen has no "Open in Stellar Expert" link for the current contract — after entering a contract ID, users have no one-click path to view the contract on the block explorer.

Solution

1. On mount, pre-fill contractId from localStorage["sorokit-recent-contracts"][0] if available.
2. Add a "Saved Contracts" dropdown below the Contract ID input listing the 5 recent IDs; clicking one sets it as the current contract.
3. Add an "Open in Stellar Expert" icon link next to the Contract ID input when contractId.trim() is non-empty.

Acceptance Criteria

• Last-used contract ID is pre-filled on mount
• "Saved Contracts" dropdown lists and applies saved contract IDs
• "Open in Stellar Expert" link appears for non-empty contract IDs
• npm run build passes

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Note for Contributors: Write a clear PR description. Include a screenshot of the saved contracts dropdown and the Stellar Expert link.