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

[FuDesign2008]

Summary

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 turned `box-shadow: 0.1em 0.1em currentColor` into an invisible white shadow on the white page.

Change

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

Button	Before	After
`--md`	`#0a84ff` bg + `#fff` text + custom `:active`	`#cfe8ff` bg, everything else inherits `button{}`
`--zip`	`#00c853` bg + `#fff` text + custom `:active`	`#c8f7da` bg, everything else inherits `button{}`

`border-color`, `color`, `box-shadow`, and the `:active` yellow now come from the baseline, so the three buttons (`← Go back` + two downloads) share the same visual grammar.

Accessibility

Black text contrast against the new backgrounds (via WCAG relative-luminance formula):

• `#000` on `#cfe8ff` ≈ 17.0:1 (was ≈3.5:1 white-on-blue)
• `#000` on `#c8f7da` ≈ 18.4:1 (was ≈3.1:1 white-on-green)

Both far exceed the WCAG AA 4.5:1 threshold for normal text.

Scope

• Only `style.css` is touched (+4 / -18 lines).
• `src/**`, `index.html`, `package.json` are untouched.
• `index.js` is byte-identical to `main` (CSS changes don't affect the JS bundle).

Verification

• `npm run typecheck` ✓
• `npm run lint` ✓ (0 error / 0 warning)
• `npm run test:run` ✓ (13 passed)
• `npm run build` ✓ (produces the same `index.js` as `main`; CI's `git diff --quiet -- index.js` will pass)
• Local dev server (`npm run start`): three buttons now share border, black offset shadow, and yellow `:active` feedback; download buttons carry a subtle blue/green tint.

Test plan

• CI green
• After merge + re-deploy, verify https://fudesign2008.github.io/clipboard-inspector/ renders the three buttons with unified style

Made with Cursor