Add warning for mentees applying to mentors with 0 slots

[Madhawa97]

Is your feature request related to a problem? Please describe.
Implement a warning message for mentees when they attempt to apply to mentors who have 0 available slots. This will inform mentees that these mentors might not be accepting new mentees.

Describe the solution you'd like

• Identify mentors with 0 available slots in the mentor listing
• Display the warning message when a mentee attempts to apply to a mentor with 0 slots
• Ensure the warning is clear and visible, but does not prevent application if the mentee chooses to proceed
• Test the feature with different scenarios (mentors with and without available slots)
• Update relevant documentation for mentees about this new feature

Describe alternatives you've considered

Additional context
Consider adding a small visual indicator (e.g., an icon) next to mentors with 0 slots in the mentor listing to make it more apparent before the mentee attempts to apply.