Enhancement os aware dark/light theme persistence

[amoduenechojo]

#closes
#166

The theme experience now behaves as requested:

The app checks the user’s OS preference with matchMedia on initial load.
A manual theme choice is saved to localStorage and takes precedence over the OS default.
A blocking inline script applies the theme class before React hydrates, preventing the opposite theme from flashing.
What changed
Added browser-side theme initialization and persistence logic in themeService.ts
Updated the theme hook in useTheme.ts to initialize from stored/local preference first, then fall back safely
Wired the pre-hydration theme script into layout.tsx
Kept the theme provider aligned in providers.tsx
Added regression coverage in useTheme.test.ts

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