Flappy bird kivy

[Ayush3941]

Fix: Corrected gravity and animation bugs in Kivy port of Flappy Bird

Changes:

• Fixed gravity direction to match Kivy’s bottom-left coordinate system (negative gravity, positive flap velocity).
• Reworked bird position tracking (bird_y) for cleaner physics updates.
• Adjusted pipe rendering and collision boxes for Kivy’s origin.
• Clamped dt to prevent jumpy animation on lag spikes.
• Fixed spawn timer and scoring logic for consistency.
• Cleaned drawing order for background, pipes, ground, and bird.

Result:
Game physics now match the original Tkinter version, animations are smooth, and gameplay feels correct.