Add Bot/Community PR categorization to digest pages

[prozolic]

Summary

This PR add this following feature.

• Add bot PR detection and heading tracking in the analyzer, classifying PRs as Bot or Community based on username patterns ([bot] suffix, @Copilot)
• Display bot PR count as a new stat card on the index page alongside total PRs and label count
• Add a "カテゴリ別" (Category) tab to daily digest pages, showing Community PRs and Bot PRs in separate sections
• Extract AppendHeadingListItem helper to share PR heading rendering logic between label view and category view

Related PR

#1

[Copilot]

Pull request overview

This PR enhances the PR digest pages by adding bot/community PR categorization functionality. It analyzes PR authors to distinguish between bot PRs (those with "[bot]" suffix or "@copilot" in the username) and community PRs, displaying them in a new "カテゴリ別" (Category) tab alongside the existing list and label views.

Changes:

• Added bot detection logic that categorizes PRs based on username patterns
• Introduced a bot PR count stat card on the index page
• Created a new category tab view on daily digest pages separating Community and Bot PRs
• Extracted a reusable AppendHeadingListItem helper method to reduce code duplication

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
src/PRDigest.NET/PullReqeustAnalayzer.cs	Added bot detection logic, PR categorization tracking, and new fields to AnalayzerResult for bot/community PR headings
src/PRDigest.NET/HtmlGenereator.cs	Added bot PR stat card, category tab UI, extracted AppendHeadingListItem helper, and updated stats grid to 3 columns

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[Copilot]

There's an inconsistency in the nullability of BotPullRequestHeadings (nullable) versus CommunityPullRequestHeadings (non-nullable). Both are initialized as new List instances and used in similar ways. Since CommunityPullRequestHeadings is also used to create a ReadOnlySpan via CollectionsMarshal.AsSpan (which requires a non-null list), it makes sense for it to be non-nullable. For consistency, BotPullRequestHeadings should also be non-nullable, and the null-coalescing operators on lines 204 and 208 in HtmlGenereator.cs can be removed.

Suggested change

	public List<HeadingBlock>? BotPullRequestHeadings;
	public List<HeadingBlock> BotPullRequestHeadings;

[Copilot]

The stats grid now has 3 columns on all screen sizes. On mobile devices (max-width: 768px), this may result in cramped cards. Consider adding a responsive style in the @media (max-width: 768px) section to make the grid single-column or wrap to 2 columns on mobile, for example: '.stats-grid { grid-template-columns: 1fr; }' or '.stats-grid { grid-template-columns: repeat(2, 1fr); }'.