refactor: remove dimmer overlay, add cyan border to preview popover

ANSI colors make the overlay content readable without dimming. Removed
preview-dimmer, liftHoveredTile(), tile--previewing/item--previewing —
all dead code now. Added var(--accent) 2px border to .preview-popover
for visual separation.

muxplex/frontend/style.css

@@ -967,7 +967,7 @@ body {
   position: fixed;
   z-index: 500;
   background: var(--bg-secondary);
-  border: 1px solid var(--border);
+  border: 2px solid var(--accent);
   border-radius: 8px;
   padding: 16px 20px;
   /* Cover most of the viewport — centered with margin */
@@ -991,19 +991,7 @@ body {
   overflow-x: hidden;
 }
 
-.preview-dimmer {
-  position: fixed;
-  inset: 0;
-  z-index: 499;              /* behind popover (500), above everything else */
-  background: rgba(0, 0, 0, 0.55);
-  pointer-events: none;       /* clicks pass through — don't block mouseleave */
-}
 
-.session-tile.tile--previewing,
-.sidebar-item.item--previewing {
-  position: relative;
-  z-index: 500;               /* same as popover — above dimmer */
-}
 
 /* ============================================================
    Responsive overlay sidebar at <960px