Persist window opacity slider value across sessions

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What is this about?

Problem

The opacity slider defaults to 96% every launch. User's last choice is never saved.

Where to look

• index.html — setOpacity() (~1476), persistState() / initApp()
• Optional: main.js — set-opacity IPC (~173)

Acceptance criteria

• Last opacity restored on startup (slider UI + window.cipher.setOpacity)
• Invalid stored values fall back to default (0.96)
• Works across hide/show and full restart

Skills needed

Same localStorage pattern already used for activeTab and messages — easy follow-on after Issue 1.