feat(home): crypto listings screen with logos, MMKV cache and component conventions

[gedu]

Closes #1

Summary

• Builds the Home screen with a crypto listings FlatList, BTC hero section, and per-coin logo images
• Caches coin logo URLs in MMKV (v4 API) to avoid redundant network requests
• Replaces deprecated ThemedText / ThemedView with project-owned Text / View primitives
• Pins Node to 20.15.0, adds iOS bundleIdentifier + Android package, and adds react-native-nitro-modules as peer dep for MMKV v4

Changes

File / Area	What changed
app/(tabs)/index.tsx	Home screen — FlatList coin list, BTC hero, API key validation
components/	CoinRow, CoinLogo, HeroSection — top-level components, props-driven
hooks/useCoinLogos.ts	Fetches + caches logo URLs in MMKV (persist across cold starts)
storage/	Migrated to MMKV v4 API (createMMKV, remove)
components/text.tsx	Replaces deprecated ThemedText (deleted)
components/view.tsx	Replaces deprecated ThemedView (deleted)
app.json	Added bundleIdentifier (iOS) and package (Android)
package.json	Scripts → expo run:*, added react-native-nitro-modules
.nvmrc	Pinned to 20.15.0
.claude/skills/react-native/SKILL.md	Documented 3 new conventions (see below)

Conventions documented in this PR

Three new rules added to the React Native skill — all applied in the code above:

1. Component Definition Location — components are always top-level in their own file, never defined inside another component body (React Compiler identity + testability)
2. FlatList Pure Callbacks at Module Level — keyExtractor / getItemLayout defined at module scope, not inside the component
3. Percentage Heights in Scroll Containers — height: '25%' resolves to 0 inside FlatList / ScrollView; use useWindowDimensions instead

Test plan

• expo run:ios — home screen loads, coins render with logos
• Kill and reopen app — logos load instantly from MMKV cache (no flicker)
• Remove API key — validation error state shown correctly
• Dark / Light mode toggle — themed primitives render correctly

📸 Screenshot: add a device screenshot here before merging.