fix: remove gradient fades, fix sidebar active indicator full-height

Remove tile-body::before and sidebar-item-body::before gradient overlays
that obscured ANSI colors at the bottom of previews. Replace box-shadow
inset with border-left on sidebar-item--active so the cyan accent stripe
spans the full item height (was only showing on the header).

muxplex/frontend/style.css

@@ -251,18 +251,7 @@ body {
   margin: 0;
 }
 
-/* Fade gradient at bottom of tile body */
+
-.tile-body::before {
-  content: "";
-  position: absolute;
-  bottom: 0;
-  left: 0;
-  right: 0;
-  height: 40px;
-  background: linear-gradient(to bottom, transparent, #000000);
-  pointer-events: none;
-  z-index: 1;
-}
 
 /* ============================================================
    Empty state
@@ -491,7 +480,7 @@ body {
 .sidebar-item--active {
   background: var(--bg-surface);
   border-color: var(--accent);
-  box-shadow: inset 3px 0 0 var(--accent);
+  border-left: 3px solid var(--accent);
 }
 
 .sidebar-item-header {
@@ -523,21 +512,7 @@ body {
   background: #000000;
 }
 
-.sidebar-item-body::before {
-  content: "";
-  position: absolute;
-  bottom: 0;
-  left: 0;
-  right: 0;
-  height: 32px;
-  background: linear-gradient(to bottom, transparent, #000000);
-  pointer-events: none;
-  z-index: 1;
-}
 
-.sidebar-item--active .sidebar-item-body::before {
-  background: linear-gradient(to bottom, transparent, #000000);
-}
 
 .sidebar-item-body pre {
   position: absolute;

muxplex/tests/test_frontend_css.py

@@ -355,13 +355,13 @@ def test_css_sidebar_item_hover():
 
 
 def test_css_sidebar_item_active():
-    """.sidebar-item--active must use inset box-shadow stripe instead of border-left-width to avoid layout shift."""
+    """.sidebar-item--active must use border-left for full-height accent indicator."""
     css = read_css()
     assert ".sidebar-item--active" in css
     block = _extract_rule_block(css, ".sidebar-item--active {")
     assert "background: var(--bg-surface)" in block
     assert "border-color: var(--accent)" in block
-    assert "box-shadow: inset 3px 0 0 var(--accent)" in block
+    assert "border-left: 3px solid var(--accent)" in block
 
 
 def test_css_sidebar_item_header():
@@ -714,19 +714,8 @@ def test_sidebar_item_body_pre_has_explicit_font_family():
     )
 
 
-def test_sidebar_item_body_gradient_fades_to_black():
+def test_no_gradient_fade_on_previews():
-    """.sidebar-item-body::before gradient must fade to #000000 (not theme background)."""
+    """Gradient fade overlays must be removed — they obscure ANSI colors at the bottom."""
     css = read_css()
-    block = _extract_rule_block(css, ".sidebar-item-body::before {")
+    assert ".tile-body::before" not in css, "tile-body::before gradient must be removed"
-    assert "#000000" in block or "transparent, #000" in block, (
+    assert ".sidebar-item-body::before" not in css, "sidebar-item-body::before gradient must be removed"
-        ".sidebar-item-body::before gradient must fade to #000000 to match black terminal bg"
-    )
-
-
-def test_tile_body_gradient_fades_to_black():
-    """.tile-body::before gradient must fade to #000000 (not theme background)."""
-    css = read_css()
-    block = _extract_rule_block(css, ".tile-body::before {")
-    assert "#000000" in block or "transparent, #000" in block, (
-        ".tile-body::before gradient must fade to #000000 to match black terminal bg"
-    )