Add project export/download and responsive mobile layout to Playground

[shai-almog]

Motivation

• Provide a way for users to export the current playground as a downloadable CN1 project zip.
• Improve the Playground editor/preview layout on narrow and portrait devices for a better mobile experience.
• Expose a toolbar action to trigger the export/download flow from the UI.

Description

• Added PlaygroundProjectExporter which composes a minimal CN1 project (Java source and theme CSS), zips it using zipme, and returns a Base64-encoded zip via ExportedProject.
• Added a download icon to the toolbar that calls downloadProject(), which uses PlaygroundProjectExporter.build() and triggerBrowserDownload() to either invoke browser-based blob download via the shared BrowserComponent JavaScript context or fall back to copying the Base64 payload to the clipboard.
• Implemented triggerBrowserDownload() and escapeJsString() to run safe JS that creates a Blob and triggers a file download in the browser, and added responsive layout logic in createMainContent() to present a two-tab mobile view in portrait mode while preserving the large-screen SplitPane and stacked layout fallbacks.

Testing

• Ran a local project assemble build (./gradlew assemble) which completed successfully.
• No automated unit tests were added or modified for this change.
• Manual verification was performed to confirm the toolbar button triggers the exporter and that the layout adapts between large, portrait, and stacked modes.

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Codex Task