fix: fixes theme management when page reloads due to navigation

[edfrias]

What changed

Improves light/dark theme handling in the global layout.

• Fixes initial page load and Astro internal navigation behavior where the preference could be lost or rendered incorrectly.
• Reads localStorage from HTML and immediately applies it to document.documentElement.dataset.theme and document.documentElement.style.colorScheme.
• Synchronizes the theme toggle so its aria-label always matches the current theme action.
• Reapplies the theme on Astro events (astro:before-swap, astro:after-swap, astro:page-load) to avoid flashes and preserve the user’s choice across route changes.

Official sources used to verify

Visual impact

Ensures the selected theme stays consistent across reloads and transitions, and the UI no longer briefly reverts to the wrong theme during navigation.

Checks run

• pnpm check
• pnpm build