Implement Designed Import / Export Flow

[PauloMFJ]

Changes

• New import flow inside in_front_of_canvas - a menu with file browse plus drag-and-drop upload (data-is-dragging-over state, blocks canvas gestures leaking through to tldraw) that restores the workspace from an exported JSON file.
• Update export flow with a full-canvas export radio option (EXPORT_OPTION_CANVAS) that's always offered regardless of selection and pairs with the new import menu.
• Update serialization with addition of STATE_VERSION and a typed ImportError (malformed | version-mismatch + fileVersion), driving full error-toast messaging when an import fails.
• Drop legacy DevPanel and its use_dev_mode hook, replaced by the import / export flow.
• Extract Card component from StatusEmpty - existing component was getting quite large as it was.
• Add new import icon.
• Update control-divider token to use existing $surface-muted for consistency.

Screenshots

[gemini-code-assist]

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[beets]

/gemini review

[gemini-code-assist]

Code Review

This pull request replaces the developer-only import/export panel with a user-facing import/export interface directly on the canvas controls. It introduces a new Import component with drag-and-drop support, integrates a full-canvas export option into the existing Export menu, and refactors state serialization to support version-mismatch checks and robust error handling. The review feedback suggests improving code robustness and type safety by using optional chaining for drag-and-drop file transfers, simplifying TypeScript type narrowing during JSON parsing, and adding stricter validation for the imported state's latestNodeId property.

[beets]

Thanks for the change - this looks nice! Some suggested changes below but approving early to unblock.