Update mer init to work with npm/pip installed binaries

[justrach]

Problem

When mer is installed via npm or pip, the mer init command needs to handle:

1. Detecting it's running from a wrapper-installed binary
2. Properly setting up the scaffold with all required files
3. Handling platform-specific paths correctly

Current State

The mer init command currently:

• Creates scaffold files from embedded templates
• Runs zig build to get fingerprint
• Sets up build.zig, build.zig.zon, starter pages

Required Updates

1. Binary Path Detection

// Need to detect if running from npm/pip wrapper
// npm: binary at node_modules/merjs/bin/mer
// pip: binary at site-packages/merjs/bin/mer

2. Template Resolution

• Ensure template files are found regardless of install method
• May need to check multiple locations for embedded files

3. Post-Init Message

Update the success message to show package-manager-specific next steps:

# For npm users:
npx mer init my-app  →  cd my-app && npm install && npm run dev

# For pip users:  
pip install merjs && mer init my-app  →  cd my-app && zig build serve

Acceptance Criteria

• mer init works correctly when mer is installed via npm
• mer init works correctly when mer is installed via pip
• Clear error messages if Zig is not installed
• Platform-specific paths handled correctly on Windows

Related

• npm distribution issue
• PyPI distribution issue
• Issue dx: consumer build.zig has to reach into merjs internals (src/main.zig, tools/codegen.zig, src/worker.zig) #67 (DX improvements)