fix: center "Open Settings" button on connection error page

[charlieleith]

Summary

The .settings-btn rule on the connection-error page (rendered when the BrowserView fails to load the stream URL) used left: 1/2;, which is invalid CSS — 1/2 isn't a valid <length> or <percentage>, so the declaration is discarded and left falls back to auto. The button only appeared centered by accident, via flex static-position resolution on the body's flex layout.

Replace with the standard horizontal-centering pattern:

left: 50%;
transform: translateX(-50%);

so the button is reliably centered regardless of the parent's display mode.

Where it shows up

src/main/main.js → did-fail-load handler → inline <style> for .settings-btn (the "Open Settings" button rendered on the dark "Failed to connect" page).

Test plan

• pnpm lint — no new warnings on src/main/main.js
• pnpm prettier --check src/main/main.js — passes
• Rendered the error-page HTML in a browser before/after; button is now positioned via valid CSS instead of relying on flex fallback behaviour.