feat(ui): full-height left sidebar with decoupled header layout

Rework the desktop layout so the left sidebar spans the full window
height and reads as a distinct column, separate from the header.

- Persistent top-left controls (sidebar toggle + project actions) as a
  single overlay that no longer migrates between header and sidebar, so
  it stays put while panels animate. Drag regions carve out only under
  the buttons so the strip stays draggable and the buttons clickable.
- Header sits in the content column (bg-background); the divider lives
  on the chat content (border-t) so it doesn't run between header and
  right sidebar. Right sidebar gets a top border + small content padding.
- Drop the rounded chat card and corner masks for flat 1px borders.
- Right sidebar, context panel and git/files views inherit the
  background color instead of bg-sidebar.
- Remove hover background from Changes/Staged rows.
- Mini-chat header: borderless, bg-background.

packages/ui/src/components/views/git/ChangeRow.tsx

@@ -118,7 +118,7 @@ export const ChangeRow = React.memo<ChangeRowProps>(function ChangeRow({
 
   return (
     <div
-      className={`group flex items-center gap-2 py-1.5 hover:bg-sidebar/40 cursor-pointer ${rowPaddingClassName ?? 'px-3'}`}
+      className={`group flex items-center gap-2 py-1.5 cursor-pointer ${rowPaddingClassName ?? 'px-3'}`}
       role="button"
       tabIndex={0}
       onClick={onViewDiff}

packages/ui/src/components/views/git/ChangesPanel.tsx

@@ -357,7 +357,7 @@ export const ChangesPanel: React.FC<ChangesPanelProps> = ({
       const isDirectoryReverting = isRevertingAll || directoryPaths.some((path) => revertingPaths.has(path));
       return (
         <div
-          className={cn('group flex items-center gap-2 py-1.5 hover:bg-sidebar/40', ROW_PADDING_CLASSNAME)}
+          className={cn('group flex items-center gap-2 py-1.5', ROW_PADDING_CLASSNAME)}
           style={{ paddingLeft: `${depth * TREE_INDENT_PX}px` }}
         >
           <button