Fix: Share dialog invisible in fullscreen mode

[Gslmao]

📌 Purpose

The share dialog was invisible when the PDF viewer was in fullscreen. Radix's DialogPortal defaults into document.body, but the browser's fullscreen API creates an isolated stacking context and only DOM nodes inside the fullscreen element are visible. So the dialog was rendering, just outside the visible fullscreen layer.

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Corresponding issue: closes #444

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🖼️ Showcase

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🔧 Changes

• dialog.tsx — added a container prop to DialogContent, passed it to DialogPortal so the render target is controllable from outside
• ShareButton.tsx — added isFullscreen and viewerRef props; passes viewerRef.current as the portal container when in fullscreen, falls back to document.body otherwise
• PDFViewer.tsx — passes isFullscreen and viewerRef down to ShareButton

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