PlotLink: Enhance book spine with page-thickness lines

[realproject7]

Summary

Follow-up to PR #291 (#208). The current card spine is a single 3px bar. Enhance it to match the book silhouette reference — a front cover with a visible spine edge and layered page lines on the left and bottom.

Reference shape (book silhouette)

The reference shows a book viewed from the front at a slight angle:

• Spine: a thick vertical line on the left edge
• Page layers: 2-3 thin parallel lines offset slightly to the left/bottom of the main cover rectangle, creating a 3D "closed book" effect — like you can see the stacked pages from the side and bottom
• Top edge: a thin horizontal line across the top suggesting the page block thickness
• The overall shape is a rectangle with layered offset lines on the left and bottom edges

Current

• Single 3px bg-border div on the left edge only

Desired

• Main card rectangle = the front cover
• Add 2-3 thin offset lines on the left side and bottom edge, slightly shifted, to create a stacked-pages / book-depth illusion
• Optionally add a thin top-edge line for the page block
• All lines are vector/stroke only — no fills, no images, no shadows
• Keep it subtle — should enhance the book feel without crowding content

Acceptance Criteria

• Card has layered offset lines on left and bottom suggesting page thickness (like a closed book)
• All lines are vector/stroke, no fills or images
• Card content still has proper padding and is not crowded
• Looks good at all responsive card sizes
• Existing spine line is incorporated into the new layered effect

Labels

agent/T3, plotlink, ui