Decouple: EC-branded copy in app screens

[chubes4]

Coupling

Hardcoded "Extra Chill" branding in screen copy:

• app/onboarding.tsx:114 — "Welcome to Extra Chill!"
• app/(drawer)/feed.tsx:63 — "Welcome to Extra Chill"
• Likely other screens (this issue covers a sweep, not just these two lines)

Target shape

Two layers:

1. Shell-owned screens (generic feed, post detail, post list) take brand strings from `config.brand` — `name`, `tagline`, optional `welcomeMessage`, etc.
2. Consumer-owned screens (EC's onboarding, EC's feed override if any) live in the consumer app and use whatever copy makes sense.

// extrachill.config.ts (consumer side)
export const config: WPNativeConfig = {
  brand: {
    name: 'Extra Chill',
    tagline: 'Online Music Scene 🥶',
    welcomeMessage: 'Welcome to Extra Chill!',
  },
  ...
};

Acceptance

• Sweep all app/** and src/** files for hardcoded brand strings
• Shell-owned strings come from config.brand.*
• Consumer-owned strings can be whatever they want
• No "Extra Chill" anywhere in shell package code

Related

Part of the wp-native extraction effort. See chubes4/wp-native ROADMAP M5/M7.