fix: website bugs

[Hofer-Julian]

I spawned a couple of subagents to find bugs in the website.
It fixed the following (smallish) bugs:

• Clicking a copy button resized it and shifted its neighbours; the BibTeX one had no confirmation style at all
• The last terminal line on the landing page was accent blue for no good reason
• The landing page hid the theme switcher and social links on phones (no sidebar drawer to house them)
• Broken code fences in the FAQ swallowed the glibc note and the whole "Why ROS and Conda?" section
• The custom fonts never reached the docs pages (--sl-font was never set)
• The sidebar "Packages" entry lost its highlight on 7 of 8 distro pages
• The copy button overlapped clipped text in the long Contributing code blocks
• Small fixes: conda page title casing, Discord link phrasing

Package table bugs

As expected, it found the most bugs in the TS code for the package table:

• Switching the mutex reset the search box, sort select and keyboard focus
• A filter could survive a mutex switch with its chip gone, silently showing "No matches"
• Scrolling rebuilt every row, killing hover state and text selection
• rosdistro repo URLs went into href unescaped
• The fetch-error fallback linked to a wrong channel URL for foxy and galactic
• The table was invisible to screen readers as a matrix: no scope="col", no text on the availability marks, no row count
• The ros-<distro>- prefix failed WCAG AA contrast and ellipsized the actual package name on phones

It suggested to use Svelte instead of plain TS code. After asking why Svelte instead of plain TS or React, that's what it said:

• Why not plain TS: every control rebuild had to carry state, focus and escaping by hand, which is exactly where the bugs above came from. Declarative rendering removes that bug class instead of patching each instance
• Why Svelte instead of React:
  • No VDOM and fine-grained updates suit a scroll-windowed table; keyed rows are patched, not rebuilt
  • Compiled output with Svelte is ~10-15 KB gzipped vs ~40+ KB for React
  • Single-file components with scoped styles match how the .astro components are already written, and the compiler emits a11y warnings at build time