feat: move projects to sidebar rail and speed up session switching (#506)

* feat: move projects from header tabs to left sidebar rail

* refactor: remove variant from git generation session context

* feat: implemented drandrop action for navrails

* refactor: improve sidebar drag-and-drop smoothness and remove floating overlay

* fix: prevent mobile nav rail touch from closing drawer and opening menu

* fix: hide header tabs on desktop

* fix: prevent session flicker when switching projects

* style: soften chat panel divider borders

* style: improve icon contrast across core navigation

* fix: stabilize session selection during project switching

* style: unify sidebar surfaces and transparent section layers

* style: remove UI shadows and keep only scroll shadow

* perf: speed up project switching with cached session loading

* perf: speed up Git changes view and background refresh

* perf: make session switching lighter and less aggressive

* perf: optimized sessions list loading while project switching

* perf: smooth chat rendering and reduce interaction spikes

* fix: restore reliable load older messages visibility

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

@@ -285,7 +285,7 @@ export const GitHeader: React.FC<GitHeaderProps> = ({
 
   if (useTwoRowHeader) {
     return (
-      <header className="@container/git-header border-b border-border/40 px-3 py-2 bg-background">
+      <header className={`@container/git-header border-b border-border/40 px-3 py-2 ${isSidebarMode ? 'bg-transparent' : 'bg-background'}`}>
         <div className="flex items-center justify-between gap-2 min-w-0">
           <div className="min-w-0 flex-1">
             {isWorktreeMode ? (
@@ -320,7 +320,7 @@ export const GitHeader: React.FC<GitHeaderProps> = ({
   }
 
   return (
-    <header className="@container/git-header flex items-center gap-2 border-b border-border/40 px-3 py-2 bg-background">
+    <header className={`@container/git-header flex items-center gap-2 border-b border-border/40 px-3 py-2 ${isSidebarMode ? 'bg-transparent' : 'bg-background'}`}>
       <div className="flex min-w-0 flex-1 items-center gap-2 overflow-hidden">
         {isWorktreeMode ? (
           <WorktreeBranchDisplay