Fix file path encoding edge cases in HTML export

[Tyrrrz]

Closes #1324

PR Overview

This pull request aims to fix edge cases in file path encoding for HTML exports. The changes introduce a new utility function to encode file paths and update the HTML export logic to use this function.

• Introduces the Url.EncodeFilePath method in the Utils namespace.
• Updates ExportContext to use Url.EncodeFilePath instead of manually formatting file URIs.

Reviewed Changes

File	Description
DiscordChatExporter.Core/Utils/Url.cs	New helper for encoding file paths with normalization.
DiscordChatExporter.Core/Exporting/ExportContext.cs	Updates file path formatting logic for HTML exports.

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

DiscordChatExporter.Core/Utils/Url.cs:18

• The char array literal syntax is invalid in C#. Please change [':', '/', ''] to new char[]{':', '/', ''}.

var separatorIndex = filePath.IndexOfAny([':', '/', '\'], position);

DiscordChatExporter.Core/Exporting/ExportContext.cs:137

• The previous logic formatted the file URI with conditional handling for relative paths. Please confirm that using Url.EncodeFilePath directly meets the intended behavior for both relative and absolute paths in HTML exports.

return Url.EncodeFilePath(optimalFilePath);