WIP: Add support for per-side, symmetric, and uniform borders in BorderWidth

[ketsebaoteth]

Changes:

• added per-side border widths (top/right/bottom/left)
• added symmetric borders (horizontal/vertical)
• added uniform borders
• updated layout/geometry to handle these
• added paint method to BorderWidth

Reason:
main goal is flexibility being able to style only the borders you care about instead of all sides at once.

Why is paint here:
without a paint method in BorderWidth, every widget that needs to draw borders would end up with a big if/else mess to handle 4 sides, symmetric cases, and uniform cases. putting the paint logic inside BorderWidth keeps that complexity in one place and avoids blotting the codebase with repeated drawing logic.

Notes:
i’m new to the codebase, so if this doesn’t fit the design direction i’m open to adjusting. border_color will follow in a separate change.

[PoignardAzur]

First, quick question, did you vibe-code this? Some of the comments make me this some of this code was AI-generated.

Regardless, why did you immediately close this PR? It seems interesting to me.