feat(about): add the About page

[ronjalh]

Add the About page

Closes #146

The About page was a placeholder. It now introduces the site, explains the sport for people who do not know gymnastics, and says where the data comes from.

What the page says

Part	Content
Our mission	Make the knowledge of the Code of Points PDF available for search and for calculating scores, for trainers and their gymnasts
Gymnastics in two minutes	A glossary: apparatus, routine, element (with the full A–J values), vault (why it has no letter), D-score, E-score, final score, Code of Points
The Code of Points and our data	Link to the official PDF (about 24 MB) and to the rules page of FIG, that we do not own the content, that the project is not affiliated with FIG, and that the catalog may not have the latest changes
For trainers	That the site is made especially for trainers and why there is a login, that the login is a test login, and what the calculator does not support yet (composition requirements, connection values, bonuses, the E-score)

Choices

• Very simple: plain text with links. No cards and no buttons of its own.
• Light and dark mode: only theme colors, so both work.
• The facts were checked against the PDF: the A–J values, that the D-score on vault is the value of the vault, the 0.20 dismount bonus, and the final score. The PDF link was checked: it answers with a PDF.
• src/lib/codeOfPoints.ts holds the two links, so they are in one place.
• global.css: .about-page is added to the two heading rules that the Elements and gymnasts pages use, so the headings look the same.

Testing

• Component tests (src/tests/pages/AboutPage.test.tsx): title, one h1 and h2 for each part, sections named by their headings, the PDF link, the rights text, the trainers and login text, links, and that there are no buttons.
• tsc -b, ESLint and all 328 frontend tests pass.
• axe-core: no violations in light and dark mode, and no horizontal scroll at 320 px.

Not in this PR

• The README still calls the page a placeholder (issue Update docs #148).
• A screen reader was not used; that has to be done by a person.
• global.css can conflict a little with PR feat(gymnasts): add the gymnast page #163, which edits the same heading rules. Keep both sides.

🤖 Generated with Claude Code

[anettkva]

Nice, clear page, and the facts hold up: I checked them against our data (the digit after the dot is the difficulty for all 749 elements, and vault goes from 1.6 to 6.4). tsc, lint, Prettier, the build and all 321 frontend tests pass, and it reads well in dark mode.

One thing: the "Element" text uses the word "grade" ("The grade is a letter…", "The code tells the grade"), which is not used anywhere else on the webpage. Could it say "difficulty" instead, like the rest of the webpage?

Generated with Claude Code, from my review.