Implement Initial Export Panel

[PauloMFJ]

Overview

Adds a floating Export panel to the Atlas canvas, triggered from the existing Export control.

What changed

• Export panel — new dropdown (export_panel.tsx) rendered in the InFrontOfTheCanvas slot: header, a status card, and an "Export options" grid (API Request Code, CSV, AI Narrative, AI Infographic, SVG, PNG). Opens with a prefers-motion-gated fade/scale and stops pointer/wheel events so canvas gestures don't fire underneath.
• Selection-aware empty states — derives its state live from the canvas: no cards ("Create a prompt to begin"), cards but none selected ("No cards selected" + a Select All shortcut), or ready ("N cards selected").
• Export context — ExportProvider / useExport (open/close/toggle) colocated in one file, wrapping the canvas and page so the panel can be opened from anywhere; the Export button in controls.tsx now toggles it with aria-haspopup/aria-expanded.
• New icon primitives — close, select, shapes.

Screenshots

[gemini-code-assist]

Code Review

This pull request introduces an export panel feature for the Atlas canvas, allowing users to export selected cards in various formats. It adds the ExportPanel component, an ExportProvider context, and integrates export actions into the canvas controls and card shapes. Additionally, typography mixins and color tokens have been updated. The feedback focuses on performance and UX improvements: splitting the export context into state and actions to prevent unnecessary card re-renders, optimizing the selected card count calculation, stopping keyboard event propagation to avoid triggering canvas shortcuts, restricting 'Select All' to card shapes, and making the shortcut label platform-aware.

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[nick-nlb]

Thanks for this as well Paulo! I've put in a couple of nits, but otherwise approved.

One of the items out of the last meeting was that the "status" section of the export (where it says "No cards selected") is functionally useful but should be smaller. We can put that in for a separate PR (along with the other deferred item mentioned in the comments.

We should make sure to track the various differed items that come out of this and other PRs so that we don't lose sight of them.