Show clear message for unsupported file types in preview pane

[Copilot]

Opening non-markdown files (e.g., .tsx, .js) displayed confusing "broken" preview content. Preview now shows an informative message for unsupported file types.

Changes

• Document model: Added isPreviewSupported computed property checking file extension against supported markdown types (.md, .markdown, .mdown, .mkd)
• Preview UI: Added PreviewUnsupportedView component displaying file type and keyboard shortcut hint
• Tests: 7 test cases covering markdown files, non-markdown files, untitled documents, and edge cases
• Docs: Updated preview.md and FAQ.md with file type support information

Implementation

// Document.swift
var isPreviewSupported: Bool {
    guard let fileURL = fileURL else { return true }  // Untitled docs supported
    let ext = fileURL.pathExtension.lowercased()
    return ["md", "markdown", "mdown", "mkd"].contains(ext)
}

// PreviewView.swift
if !appState.currentDocument.isPreviewSupported {
    PreviewUnsupportedView(fileExtension: fileURL?.pathExtension ?? "")
} else {
    MarkdownWebView(html: renderedHTML, baseURL: resourceBaseURL)
}

Preview behavior unchanged for markdown files.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve UI: file types that can't be previewed should have preview disabled</issue_title>
<issue_description>Example:

A user opens a .tsx file. The Preview appears "broken", but it's actually just not supported.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve UI: file types that can't be previewed should have preview disabled #34

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