fix: correct border color to #2E2E31, default item height to 20, fix comment typo

- CortexSidebarContainer: border #2E2F31 → #2E2E31 (matches Figma stroke)
- VirtualFileTree: DEFAULT_ITEM_HEIGHT 18 → 20 (matches Figma tree row)
- ExplorerHeader: fix padding comment (top=4 right=4 bottom=12 left=4)
- FIGMA_SPECS.md: fix file padding formula in docs

Author: echobt

src/components/cortex/FIGMA_SPECS.md

@@ -781,7 +781,7 @@ File type icon: `16×16` (inside tab)
 | Align Items | `center`                      | `center`                     |
 | Height      | `20px`                        | `20px`                       |
 | Gap         | `8px`                         | `4px`                        |
-| Padding     | `0 8px 0 {indent}`            | `2px 8px 2px {indent+28}`   |
+| Padding     | `0 8px 0 {indent}`            | `0 8px 0 max(0,level-1)*26+28` |
 | Sizing H    | `fill`                        | `fill`                       |
 
 ### Indentation

src/components/cortex/explorer/ExplorerHeader.tsx

@@ -2,7 +2,7 @@
  * ExplorerHeader - Tab header for Explorer panel
  * Figma: pill-in-container tab switcher (file 4hKtI49khKHjribAGpFUkW, node 1060:33326)
  *
- * Outer frame: 320×48, padding 4 4 4 12
+ * Outer frame: 320×48, padding top=4 right=4 bottom=12 left=4
  * Tab bar: 312×32, bg #141415, border-radius 8px, padding 4, gap 4
  * Active button: 150×24, bg #1C1C1D, border-radius 4px
  * Inactive button: 150×24, bg transparent, border-radius 4px

src/components/cortex/layout/CortexSidebarContainer.tsx

@@ -76,7 +76,7 @@ export function CortexSidebarContainer(props: CortexSidebarContainerProps) {
     transition: props.isResizing ? "none" : "width 150ms ease, opacity 150ms ease",
     background: "#1C1C1D",
     "border-radius": "12px",
-    border: "1px solid #2E2F31",
+    border: "1px solid #2E2E31",
     opacity: props.sidebarCollapsed ? "0" : "1",
     display: "flex",
     "flex-direction": "column",

src/components/explorer/VirtualFileTree.tsx

@@ -57,7 +57,7 @@ export interface VirtualFileTreeProps {
   onOpen?: (path: string) => void;
 }
 
-const DEFAULT_ITEM_HEIGHT = 18;
+const DEFAULT_ITEM_HEIGHT = 20;
 const DEFAULT_OVERSCAN = 10;
 const LAZY_LOAD_DEPTH = 1;