Export dialog → compact export mini-bar; improve export sizing and navigation handling

[Manethpak]

Motivation

• Replace the blocking export dialog with a compact, non-modal export mini-bar that surfaces format selection and quick actions while improving responsive layout and visual polish.
• Improve browser history and navigation semantics so importing the first image pushes a view state and clearing an image restores the URL and responds to the back button.

Description

• Reworked ExportDialog into a compact export-mini-bar UI, removed the base64 textarea, and switched to format chips plus inline actions for Download, Copy image, and Copy base64, while adding an async preview size calculation using previewSize and isCalculatingSize.
• Updated App.tsx to add imageRef for stable image existence checks, push #editor on first import, toggle export visibility instead of always opening, convert clearImage to a useCallback that synchronizes the URL, and listen for popstate to clear the image on back navigation.
• Updated styles in src/App.css to add .export-mini-bar layout, animations, responsive rules, adjust .format-chip styling, and tweak .preview__bar grid-template for consistent layout.
• Removed unused refs and backdrop/escape-close handlers tied to the old modal export dialog and simplified some spacing/typography for the new bar.

Testing

• Ran TypeScript typecheck with npm run typecheck and a production build with npm run build, and both succeeded.

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Codex Task