fix: remove all gaps between masonry cards

- column-gap: 0, margin-bottom: 0 → images are flush edge-to-edge
- Replace border with inset box-shadow for hover/selected (no layout cost)
- Remove grid-pane padding so grid fills the container fully

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/pixiv_dialog.css

@@ -89,30 +89,28 @@
 .px-grid-pane {
   flex: 1;
   overflow-y: auto;
-  padding: 8px;
+  padding: 0;
 }
 
 .px-grid {
   column-width: 128px;
-  column-gap: 2px;
+  column-gap: 0;
 }
 
 /* Cards */
 .px-card {
   position: relative;
   cursor: pointer;
-  border-radius: 4px;
   overflow: hidden;
-  border: 2px solid transparent;
   background: #181825;
-  transition: border-color 0.15s;
+  transition: box-shadow 0.15s;
   break-inside: avoid;
   display: inline-block;
   width: 100%;
-  margin-bottom: 2px;
+  margin-bottom: 0;
 }
-.px-card:hover { border-color: #7f849c; }
-.px-card.selected { border-color: #cba6f7; }
+.px-card:hover  { box-shadow: inset 0 0 0 2px #7f849c; }
+.px-card.selected { box-shadow: inset 0 0 0 2px #cba6f7; }
 
 .px-card img {
   width: 100%;
@@ -406,7 +404,7 @@
 .px-artist-grid-scroll {
   flex: 1;
   overflow-y: auto;
-  padding: 8px;
+  padding: 0;
 }
 
 /* Misc */