Clarify attention output projection

[cat0825]

Summary

• Clarify that each attention head output is concatenated and then passed through GPT-2's attention output projection matrix (c_proj).
• Update the article and guided textbook copy so learners understand why c_proj is present in the code but not drawn as a separate matrix.
• Fix the attention output tooltip to refer to the selected head instead of always saying Head 1.

Fixes #71

Verification

• npx prettier --check src/components/Attention.svelte src/components/article/Article.svelte src/utils/textbookPages.ts
• git diff --check
• npm run check could not complete because the current project baseline reports 499 existing errors unrelated to this PR.
• SASS_PATH=. npm run build compiles the app bundles, then fails during prerender because sharp was installed with scripts disabled and its native binary is unavailable in this local environment.