fix(ui): unify download button style with project baseline (#4)

The `Download as Markdown` / `Download as ZIP` buttons introduced in #1
used a Material-style filled look (saturated blue/green + white text)
that clashed with the project's Neo-Brutalism language (thick black
border, light background, black text, offset black shadow, yellow
`:active`). The white `color` also made `box-shadow: 0.1em 0.1em
currentColor` render as an invisible white shadow on the white page.

Make the two download buttons inherit the baseline `button{}` rules and
only override `background` for light-tinted semantic hints:

- `--md`  → `#cfe8ff` (light blue, black text, contrast ≈17:1)
- `--zip` → `#c8f7da` (light green, black text, contrast ≈18:1)

Border color, text color, offset shadow, and `:active` yellow now come
from the baseline, so the three buttons (`Go back` + two downloads)
share the same visual grammar. Net -14 lines of CSS; no JS or markup
changes; `index.js` is byte-identical to main.

Made-with: Cursor

Author: FuDesign2008

style.css

@@ -195,30 +195,16 @@ h1 + p {
 }
 
 .download-button--md {
-	background: #0a84ff;
-	color: #fff;
-	border-color: #0060c7;
-}
-
-.download-button--md:active {
-	background: #0060c7;
+	background: #cfe8ff;
 }
 
 .download-button--zip {
-	background: #00c853;
-	color: #fff;
-	border-color: #00a043;
-}
-
-.download-button--zip:active {
-	background: #00a043;
+	background: #c8f7da;
 }
 
 .download-button[disabled] {
-	background: #ccc;
-	color: #777;
-	border-color: #999;
-	cursor: not-allowed;
+	background: #e8e8e8;
+	color: #999;
 }
 
 @media (max-width: 60ch) {