Review grid thumbnail border and selection styling

[chammoru]

Problem

The planned gallery-style grid thumbnail view (#78) will likely need visible boundaries between thumbnails. Without borders or clear spacing, adjacent images may blend together, especially when thumbnails are large or when images have similar colors.

The exact border style and thickness should be discussed before implementation.

Open design question

Need UX/design review for:

• Whether every grid thumbnail should have a border.
• Border thickness: 1 px, 2 px, or theme/DPI-aware thickness.
• Border color for normal, hover, selected, and current-image states.
• Whether spacing/gutter alone is enough, or if a visible border is required.
• How borders should look in dark/light themes.
• How raw/non-previewable items should be represented in the same grid.

Possible direction

Use a subtle default border or card-style tile boundary, then use a stronger accent border for the selected/current image. The selected state should be visually obvious without making the grid look noisy.

Relationship to other issues

This is a follow-up design detail for #78.

Acceptance criteria

• Decide whether grid thumbnails use borders, gutters, cards, or a combination.
• Define border thickness and color rules for normal/hover/selected/current states.
• Grid thumbnails remain visually separated and easy to scan.
• Selection/current-image state is clearly visible.
• Styling works acceptably on light and dark Windows themes.