Refactor export helpers into reusable node-export module

[mniinio]

Motivation

• Separate CLI orchestration from reusable export logic so programmatic consumers can import helpers without pulling in the entire CLI.
• Make the rendering/output helpers reusable for other scripts and consumers and enable a package subpath for direct imports.
• Preserve existing behavior and test surface while simplifying scripts/export.js to a thin CLI wrapper.

Description

• Added src/node-export.cjs and moved non-CLI export logic there, including buildContent, buildFileName, renderSVG, writePDF, writePNG, and re-exported exportJSON from scripts/noteManager.
• Simplified scripts/export.js to only parse CLI arguments and call the shared helpers from ../src/node-export.cjs.
• Exposed the new module via package.json exports as "./node-export": "./src/node-export.cjs" while keeping the existing root entry points intact.
• Updated tests/export.test.js to import the helpers from src/node-export.cjs and validated the same exported API surface is available.

Testing

• Ran npm test -- tests/export.test.js and the test suite passed (all tests in tests/export.test.js succeeded).
• Ran npm run build and the build completed successfully.

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Codex Task

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