feat: implement dark theme with system integration and persistent use…

[amanjn38]

Pull Request (PR) Checklist

Please check if your pull request fulfills the following requirements:

• The PR is submitted to the master branch.
• I confirm that I've run the code locally and everything works as expected.
• 🎬 I've attached a screen recording of the changes (in case of UI changes).

Tip: drag & drop the video to the PR description.

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

What's changed?

Describe with a few bullets what's new:

• Implemented comprehensive dark theme with Material Design 3 colors
• Added system theme integration - app follows phone's theme by default
• Created persistent theme preferences using DataStore
• Added animated ThemeSwitcher component with smooth transitions
• Integrated theme switcher into HomeScreen for easy user access
• Added ThemeViewModel for centralized theme state management
• Implemented robust error handling for DataStore type safety
• Support for three theme modes: SYSTEM (default), LIGHT, and DARK
• Theme preferences persist across app restarts

💡 Tip: Please, attach screenshots and screen recordings. It helps a lot!

Risk Factors

What may go wrong if we merge your PR?

• DataStore migration issues if users have existing conflicting preferences (mitigated with error handling)
• Potential UI inconsistencies in screens not tested with dark theme
• Theme switching animation might cause brief visual glitches on slower devices

In what cases your code won't work?

• If DataStore dependency fails to initialize (gracefully falls back to system theme)
• On devices with corrupted preference storage (handled with try-catch blocks)
• If Hilt dependency injection fails for ThemeDataStore (app would crash - requires proper DI setup)

Does this PR closes any GitHub Issues?

Check CollegeSearch Issues.

• Closes #ISSUE_NUMBER

Replace ISSUE_NUMBER with your issue number (for example Closes #1234). If you've done that correctly, you'll see the issue title linked when previewing your PR description.