[eric] ui: kill inner seams, app-builder header band + file-tree line + skill markdown hr gone, columns separate by tint

frontend/src/app/pages/Skills/Skills.tsx

@@ -263,7 +263,7 @@ const Skills: React.FC = () => {
           '& pre': { bgcolor: c.bg.secondary, borderRadius: `${c.radius.sm}px`, p: 2, mb: 1.5, overflow: 'auto',
             '& code': { bgcolor: 'transparent', color: c.text.secondary, px: 0, py: 0 },
           },
-          '& hr': { border: 'none', borderTop: `1px solid ${c.border.subtle}`, my: 2 },
+          '& hr': { border: 'none', my: 3 },
           '& a': { color: c.accent.primary, textDecoration: 'none', '&:hover': { textDecoration: 'underline' } },
           '& strong': { fontWeight: 600, color: c.text.primary },
         }}>
@@ -308,7 +308,7 @@ const Skills: React.FC = () => {
       <Box
         sx={{
           width: SIDEBAR_W, minWidth: SIDEBAR_W, height: '100%', display: 'flex', flexDirection: 'column',
-          borderRight: `${c.border.width} solid ${c.border.subtle}`, bgcolor: 'transparent',
+          bgcolor: c.bg.secondary,
         }}
       >
         <Box sx={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', px: 2, pt: 2, pb: 1 }}>

frontend/src/app/pages/Views/ViewEditor.tsx

@@ -1103,9 +1103,7 @@ const ViewEditor: React.FC<Props> = ({ output }) => {
             gap: 2,
             px: 1.5,
             py: 1,
-            bgcolor: c.bg.secondary,
-            // Hairline separates the meta strip from the tabs row.
-            borderBottom: `0.5px solid ${c.border.subtle}`,
+            bgcolor: 'transparent',
             flexShrink: 0,
             minHeight: 48,
           }}
@@ -1154,8 +1152,7 @@ const ViewEditor: React.FC<Props> = ({ output }) => {
           sx={{
             display: 'flex',
             alignItems: 'center',
-            // No borderBottom; bg-color step carries the separation.
-            bgcolor: c.bg.secondary,
+            bgcolor: 'transparent',
             flexShrink: 0,
             px: 1.25,
             py: 1,
@@ -1276,7 +1273,6 @@ const ViewEditor: React.FC<Props> = ({ output }) => {
                 sx={{
                   width: 200,
                   flexShrink: 0,
-                  borderRight: `1px solid ${c.border.subtle}`,
                   bgcolor: c.bg.secondary,
                   display: 'flex',
                   flexDirection: 'column',