refactor(gui): replace JPanel with AWT Canvas for rendering

refactor(gui): replace JPanel with AWT Canvas for rendering

Convert ViewPanel from Swing JPanel to AWT Canvas to use BufferStrategy
for page-flipping rendering. This provides better control over the
rendering pipeline and avoids Swing's double-buffering overhead.

Also fixes a typo in Line.java (greenWithAplha -> greenWithAlpha) and
makes Color.a field final.