feat(VFileInput): add visual drag-over feedback

[sridhar-3009]

Closes #20576

Problem

When dragging files over VFileInput, there was no visible indication that the area accepts dropped files. The isDragging state existed and the v-file-input--dragging CSS class was applied, but had no visual styling beyond a z-index change.

Fix

Added drag-over visual feedback styles:

• For all variants: a primary-colored border appears during drag
• For outlined variant: full-opacity border with primary color using --v-field-border-opacity
• Added $file-input-dragging-border-color and $file-input-dragging-opacity SASS variables for customization

Result

Users now get clear visual feedback when dragging files over the input area, similar to other file drop implementations (Quasar, etc.).

[J-Sek]

closing in favor of #22909