fix: Respect OS theme preference on first load

[Ruchika083]

Summary

Updated the frontend theme initialization logic to respect the user's OS-level prefers-color-scheme setting on first load.

Changes Made

• Use saved theme from localStorage if available
• Fallback to prefers-color-scheme when no saved theme exists
• Preserve existing manual theme toggle behavior

Files Changed

• frontend/index.html

closes #178

Attached screenshots showing the app correctly respecting OS-level theme preference on first load.

• Light OS preference → app loads light theme

• Dark OS preference → app loads dark theme

[Ruchika083]

Hi @imDarshanGK, please let me know if there are any changes needed from my side for the above.

[imDarshanGK]

@Ruchika083 Could you please attach the related issue number to this PR for proper tracking?
and Before review, please include screenshots or demo output for the implemented feature/fix.

[Ruchika083]

@imDarshanGK ,
I have updated the PR with related issue number and added screenshots showing the implementation of the fix.

[imDarshanGK]

@imDarshanGK Please fix the [] attribute regression in [index.html]. The diff changes data-theme to en, which breaks theme initialization. After correcting that, the prefers-color-scheme startup logic can be re-reviewed.

[Ruchika083]

@imDarshanGK , I have made the changes, please let me know if there are any changes needed from my side for the above.

[imDarshanGK]

The theme-preference fix looks correct and does address issue #178, but this PR is not merge-ready yet because the diff also removes unrelated backend digest files that are already on main. Please rebase this branch onto the latest main and keep only the frontend theme initialization change. After that I can approve and merge.