feat: implement ModeManager with observer pattern (#15)

[sudiptom619]

Closes #15

What was built:

• ModeManager class in core/hybrid/mode_manager.py
• switch_mode() with validation and ValueError for invalid modes
• get_current_mode() returns mode + description
• on_mode_change() registers observer callbacks
• _notify_observers() triggers all callbacks on mode change
• 7 unit tests covering all methods

Note: mode_manager instance is exported for use in api/routes/mode.py

[sudiptom619]

Hi @sahoo-tech , I'm aware GSSoC contributions officially start May 15th. This PR is ready for review whenever the window opens.

[sudiptom619]

@sahoo-tech can you please add the gssoc label to this?

[sudiptom619]

@sahoo-tech please add gssoc label to this so points get updated in the profile.