Add ARIA roles and labels to tabs and icon buttons

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What is this about?

Problem

Cipher's UI has almost no accessibility metadata. Screen reader users can't tell which tab is active or what icon-only buttons do.

Currently only one aria-hidden exists on the brand icon.

Where to look

• index.html — tab buttons (~617), titlebar buttons (~602), composer, opacity slider (~705), capture remove buttons

Acceptance criteria

• Tab list uses role="tablist"; tabs have role="tab", aria-selected, aria-controls
• Icon-only buttons (hide, close, remove capture, copy code) have aria-label
• Opacity slider has a label or aria-label

Skills needed

HTML accessibility basics — great first PR, no backend work.