Subscription store, paywall on locked units, profile subscription bar, Tadrisino branding

- Store opens with two choices (subscription / question packs); 3- and 6-month cards
  show per-month price and saving.
- Units that need a subscription show a crown badge and open the store; a 402 from
  the quiz goes there too. «ادامه بده» skips locked units.
- Profile: subscription card with an animated days-left bar (planDurationDays).
- BrandMark (SVG) on splash, home header and footer; copyright footer; floating math
  glyphs on splash. CSS transform/opacity only; honours reduced motion.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/screens/UnitScreen.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { Sparkles } from 'lucide-react';
+import { Crown, Sparkles } from 'lucide-react';
 import type { HomeNode } from '@/api';
 import { useAppData } from '@/state/AppData';
 import { findUnit, unitColor, unitProgress } from '@/state/curriculum';
@@ -14,6 +14,7 @@ import { MathText } from '@/lib/MathText';
 export function UnitScreen({ unitId }: { unitId: string }) {
   const { home, homeError, refreshHome, companion } = useAppData();
   const nav = useNav();
+  const paywall = () => nav.push({ name: 'subscription', view: 'subscription' });
   const [openNode, setOpenNode] = useState<HomeNode | null>(null);
   const found = findUnit(home, unitId);
 
@@ -58,9 +59,14 @@ export function UnitScreen({ unitId }: { unitId: string }) {
               nodes={unit.nodes}
               color={color}
               currentId={current?.id ?? null}
-              onOpen={setOpenNode}
-              onStart={(node) => nav.push({ name: 'quiz', nodeId: node.id })}
+              onOpen={(node) => (unit.requiresSubscription ? paywall() : setOpenNode(node))}
+              onStart={(node) => (unit.requiresSubscription ? paywall() : nav.push({ name: 'quiz', nodeId: node.id }))}
             />
+            {unit.requiresSubscription && (
+              <button type="button" onClick={paywall} className="mx-auto mt-6 flex items-center gap-2 rounded-2xl bg-orange px-5 py-3 text-sm font-black text-white shadow-lg shadow-orange/30">
+                <Crown size={16} /> این فصل با اشتراک باز می‌شه
+              </button>
+            )}
             <div className="mt-6 flex flex-col items-center">
               <span className="mb-1 text-[11px] font-bold text-muted">{progress.done === progress.total ? 'این فصل تموم شد!' : 'ادامه بده!'}</span>
               <span className="flex h-11 w-11 items-center justify-center rounded-2xl" style={{ backgroundColor: `${color}18` }}>