Tiles view: lazy placeholder is 4px shorter than a hydrated tile, causing layout shift #3170

Part of the file list scroll performance work for #3159.

Current situation

In the tiles view a lazy placeholder tile is 4 px shorter than a hydrated one.

Measured on a 500-image folder, tiles size 3:

• placeholder tile: 220.5625 px
• hydrated tile: 224.5625 px
• grid: 4 columns x 125 auto-sized rows

The placeholder markup in packages/web-pkg/src/components/FilesList/ResourceTile.vue does not reproduce the hydrated tile's box exactly.

Why this is bad

Every reveal grows its grid row and pushes everything below it down. While scrolling, content moves under the user's finger and the scroll position drifts. Each reveal also invalidates layout for the whole 500-item grid.

Proposed solution

1. Make the placeholder exactly the height of a hydrated tile.
2. Pin the grid row height so revealing a tile cannot resize its row.

Verification

Placeholder and hydrated tile report the same getBoundingClientRect().height. Scroll position stays stable while tiles fill in - no cumulative shift.