Fix staged screenshots not persisting across app restarts

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What is this about?

Problem

When you capture screens, stage them, then restart Cipher, the staged captures disappear.

loadState() tries to restore saved.screenshots, but persistState() never saves them.

Where to look

• index.html — persistState() (~line 1083)
• index.html — loadState() (~line 1072)

Acceptance criteria

• Staged captures survive app restart
• persistState() and loadState() use the same data shape
• App still starts cleanly if localStorage data is corrupt/too large

Skills needed

JavaScript, localStorage — no Electron main-process knowledge required.

How to test

1. npm start
2. Capture 1–2 screens (don't send yet)
3. Quit and relaunch — captures should still be there