fix: guard tagName.toLowerCase() against undefined element

[tt-a1i]

Summary

During Next.js page transitions, elements may be destroyed and element.tagName can become undefined, causing TypeError: Cannot read properties of undefined (reading 'toLowerCase').

This PR adds defensive checks to 4 places where tagName.toLowerCase() was called without protection:

• context.ts:141 - getHTMLPreview function
• context.ts:183 - formatElements helper
• agent.ts:180 - element validation check
• is-selection-inside-editable-element.ts:2 - isEditableElement function (called from selectionchange event)

Changes

- const tagName = element.tagName.toLowerCase();
+ const tagName = (element.tagName || "").toLowerCase();

Related

Fixes #44

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