Blog refresh: new design, D2Coding font and code highlighting

A full refresh of how the blog looks and works. Post text was not changed.

Home page

The home page now shows the most recent post instead of the original theme introduction.
Buttons under the post lead to the post itself (with comments) and to the full post list.

New logo: a lambda between code brackets, <λ>, where math meets code. Small favicons use a bolder version so it stays readable.
Reworked dark theme based on the existing blue and orange palette. Headings and body text now have proper contrast.
Sticky header with a working menu on mobile.
Post pages: table of contents on the left, a comfortable reading width, date and tags under the title.
Post list (Thoughts): thumbnails, tags and a two-line summary per post.
Posts without their own feature image get a default banner and thumbnail in the same style as the logo.
Notes written elsewhere can be imported as posts with their tags as categories and their line breaks kept.
Categories page: compact lists with post counts.
New Update menu for change notes like this one.

The whole site uses D2Coding by NAVER, a monospaced font that also covers Korean.
The font is served from this site, not from a third-party font service, under the SIL Open Font License 1.1 (license text).

Styles for headings, lists, blockquotes, tables, images and inline code.
Fenced code blocks get syntax highlighting per language, a language label and a copy button.
Code blocks without a language are shown as plain text, so write the language after the opening fence:

```java
System.out.println("Hello");
```

The site is built only from files in this repository.
Removed leftovers from the original theme template: the demo contact form, the custom domain setting, sponsor and contribution files, and links to the theme author’s personal pages.
The search page was rewritten and hardened.
Fixed links that ignored the /FirstBlog base path (RSS feed, offline page, installed-app start page).
The RabbitMQ post now has four separate categories. Its old address redirects to the new one.
Local builds use the same Jekyll version as GitHub Pages.