Navigation restructure: five-tab bottom bar, no phone drawer

Drawer removed (phones only; desktop rail untouched)
- MobileSidebar, sidebarOpen/useSidebarOpen, MenuIcon and the drawer's
  back-gesture layer are gone; the shared nav content now serves only
  the desktop rail (dead mobile branches deleted). No drawer-specific
  CSS existed; the three drawer strings (header.openMenu,
  sidebar.closeMenu, common.menu) and library.importAria are removed.
- The header drops the hamburger: logo + name on the left, the single
  theme chip on the right.

Five-tab bottom bar
- Library / Progress / New deck (prominent centre +) / Exam / Settings,
  in that order; Progress and Exam reuse the existing /stats and /exam
  screens, Settings keeps opening the dialog. Sliding pill, icon bounce,
  press feedback, re-tap-to-top and safe-area padding kept; every item
  is >=44px (72px wide at 360px).
- New short tab.* keys (EN + CS) so all five labels stay on one line at
  360px — 'Zkouškový režim' became 'Zkouška'. No ellipsis anywhere.
- The bar is mounted once in AppHeader so library, progress, exam and
  share get it; the wizards keep their own bottom action bar and the
  tab bar steps aside there, exactly as before.

Re-homed drawer destinations
- Smart Review card loses the Exam link (it is a tab now); Review due
  and Mistakes stay.
- New deck screen (and /homework) get a Study deck / Make my homework
  segmented switch; the library's homework empty-state button stays.
- The phone import icon becomes a ... button opening a sheet with
  Import JSON and New folder (back-gesture and Escape close it); folder
  chips still appear only once at least one folder exists, and folder
  creation stays in the New deck options step.

Verified: mobile suite 134 checks (no hamburger/drawer, tab order and
44px targets, label matrix at 360/390 x EN/CS x dark/light x reduced
motion, per-tab routing, deep links, Back from every tab, the ... sheet
with folder creation) plus regression 74/74, wizard 83/83, declutter
116/116, animation 8/8, native 35/35, i18n 25/25, phase1 38/38, phase2
43/43, phase3 20/20, freeze-loading 6/6. Key parity 740/740. Lint +
build clean. SW freezmo-v28.

app/exam/page.tsx

@@ -1,7 +1,7 @@
 import { ApiKeyGate } from "@/components/ApiKeyGate";
 import { AppHeader } from "@/components/AppHeader";
 import { ExamScreen } from "@/components/exam/ExamScreen";
-import { MobileSidebar, Sidebar, SidebarOffset } from "@/components/Sidebar";
+import { Sidebar, SidebarOffset } from "@/components/Sidebar";
 
 export const metadata = {
   title: "Exam mode",
@@ -13,7 +13,6 @@ export default function ExamPage() {
       <div className="flex min-h-dvh">
         <Sidebar />
         <SidebarOffset />
-        <MobileSidebar />
         <main className="flex min-h-dvh min-w-0 flex-1 flex-col">
           <AppHeader />
           <div className="mx-auto w-full max-w-3xl flex-1 px-4 pb-tabbar lg:px-10 lg:pb-16 lg:pt-10">