feat(site): polish right panel tabs, diff stats, and file headers (#22636)

Polishes the right panel UI introduced in #22633:

<img width="3138" height="1596" alt="image"
src="https://github.com/user-attachments/assets/d3947db0-6600-4469-b7e2-6eb80aadb7bc"
/>

Over 2k lines of this is just the Seti font definition.

The file tree view isn't actually adjusting much, it's just a scroll
helper. Soon I'll add a comment system so users can leave agents
feedback directly from the code.

.github/workflows/typos.toml

@@ -30,6 +30,9 @@ HELO = "HELO"
 LKE = "LKE"
 byt = "byt"
 typ = "typ"
+# file extensions used in seti icon theme
+styl = "styl"
+edn = "edn"
 Inferrable = "Inferrable"
 
 [files]