feat: Linear-style sidebar with explicit toggle

Rewrite sidebar from scratch: 256px expanded, 56px collapsed via
click toggle + [ keyboard shortcut. Two-phase CSS transitions (labels
fade then width contracts). Contextual ContentHeader replaces
UtilityBar (no logo, just actions). Remove framer-motion sidebar
primitive, hover-to-expand, and sidebar-context.

lib/sidebar-context.tsx

@@ -1,31 +0,0 @@
-'use client'
-
-import { createContext, useCallback, useContext, useState } from 'react'
-
-interface SidebarContextValue {
-  mobileOpen: boolean
-  openMobile: () => void
-  closeMobile: () => void
-}
-
-const SidebarContext = createContext<SidebarContextValue>({
-  mobileOpen: false,
-  openMobile: () => {},
-  closeMobile: () => {},
-})
-
-export function SidebarProvider({ children }: { children: React.ReactNode }) {
-  const [mobileOpen, setMobileOpen] = useState(false)
-  const openMobile = useCallback(() => setMobileOpen(true), [])
-  const closeMobile = useCallback(() => setMobileOpen(false), [])
-
-  return (
-    <SidebarContext.Provider value={{ mobileOpen, openMobile, closeMobile }}>
-      {children}
-    </SidebarContext.Provider>
-  )
-}
-
-export function useSidebar() {
-  return useContext(SidebarContext)
-}