fix: add Close button to up-to-date update dialog

[ibetitsmike]

The "You're up to date" state in the update dialog had no way to dismiss the window. Adds a Close button (matching the existing Retry button style) that closes the updater window.

Changes: src/lib/UpdateDialog.svelte

• Import getCurrentWindow from @tauri-apps/api/window
• Add a Close button to the up_to_date view state

Validation: svelte-check ✓ · vitest (15/15) ✓ · smoke ✓ · vite build ✓

[ibetitsmike]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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