Add frontend linting and improve type safety

[VolkannSahin]

• Add ESLint checks to frontend CI.
• Add automatic pre-commit linting with Husky.
• Replace unsafe any types with typed interfaces.
• Add reusable API error-handling helpers.
• Separate authentication context and provider.
• Correct React effect dependencies and timer cleanup.

[toladnertum]

Thanks! I merged a few other branches, which caused some merge conflicts. Happy to merge once these are resolved.