Prevent address generation on startup

[ekulkisnek]

Description

This PR resolves the issue where the Thunder GUI window size was too small on startup and caused layout squishing (vertical letters) in the Mempool Explorer and Transaction Builder tabs.

Changes Made

1. Initial Window Size: Configured eframe::NativeOptions in app/main.rs to set the default viewport inner size to 1280x720 pixels.
2. Transaction Builder Tab: Replaced nested SidePanel containers inside ScrollArea with ui.horizontal and ui.vertical columns using fixed widths. This resolves layout breaking and enables correct horizontal scrolling behavior. Also updated the spent UTXOs grid ID to "spent_utxos" to prevent ID collisions.
3. Mempool Explorer Tab: Wrapped the columns in a horizontal ScrollArea and replaced nested SidePanels with ui.horizontal and ui.vertical columns. Renamed the outputs grid from "inputs" to "outputs" to fix a duplicate ID warning.
4. Formatting: Formatted all changes using cargo fmt.

Verification

• Verified compilation with cargo check.
• Verified that the full workspace test suite passes successfully with cargo test.
• Verified that no Clippy warnings or formatting errors are present.

---

This pull request was made by Antigravity Gemini 3.5 Flash High.

[fumin]

Thanks, I think this commit c4fe7d6 in this PR solves the issue #80 where a new wallet address was generated every time the Thunder app starts up.