feat: devnet build configuration

[WiktorStarczewski]

Summary

• Adds MIDEN_NETWORK env variable to control build-time network targeting (default: testnet)
• yarn build:devnet produces a devnet extension with blue/gray branding, devnet endpoints, and "Miden Wallet (Devnet)" manifest name
• Replaces all hardcoded brand color hex values (#FF5500, #EE622F, etc.) with network-conditional constants from src/utils/brand-colors.ts
• Adds 14 devnet SVG icon variants and conditional icon selection in Settings, Onboarding, Unlock, and BiometricUnlock screens

New build scripts

• yarn build:devnet — extension
• yarn dev:devnet — extension dev watch
• yarn build:mobile:devnet — mobile
• yarn build:desktop:devnet — desktop

How it works

One env variable (MIDEN_NETWORK) drives everything:

• Webpack EnvironmentPlugin injects it into all 7 build configs
• constants.ts reads it to set DEFAULT_NETWORK → controls endpoints
• tailwind.config.ts reads process.env.MIDEN_NETWORK → controls CSS colors
• brand-colors.ts reads DEFAULT_NETWORK → controls inline/SVG colors
• webpack.public.config.js transform → controls manifest name

A plain yarn build (no env var) is unchanged — testnet by default.

Test plan

• yarn build produces testnet extension (orange, "Miden Wallet")
• yarn build:devnet produces devnet extension (blue/gray, "Miden Wallet (Devnet)")
• yarn test — 1639 tests passing
• yarn lint — clean (0 warnings)
• Load both extensions side-by-side and verify visual difference