feat: add justfile with help target

[ayomidearegbeshola29-dev]

Summary

Adds a justfile with a help target (the default recipe) that lists all available development commands for both the Soroban contracts and the Next.js frontend.

Changes

New file: justfile with recipes:

Recipe	Description
help (default)	List all recipes
build	Build Soroban contracts to WASM
test	Run contract tests
fuzz [level]	Run fuzz tests (quick/standard/extended/thorough)
check-wasm	Enforce 256 KB WASM size budget
clippy	Clippy lint with deny warnings
fmt / fmt-check	Format / check formatting
frontend-*	Install, dev, build, lint, typecheck frontend
check	Combined: fmt-check + clippy + tests + WASM + frontend

Usage

$ just
Available recipes:
    build             # Build all Soroban contracts
    check             # Run all checks (format, clippy, tests, WASM size, frontend lint + typecheck)
    check-wasm        # Check WASM binary size (256 KB budget)
    clippy            # Clippy lint (deny warnings)
    fmt               # Format all code
    fmt-check         # Check formatting without writing
    frontend-build    # Production build (includes type checks)
    frontend-dev      # Start local dev server
    frontend-install  # Install frontend dependencies
    frontend-lint     # Lint frontend
    frontend-typecheck # Type-check frontend
    fuzz              # Run fuzz tests (quick | standard | extended | thorough)
    help              # List available recipes
    test              # Run contract tests

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closes #2187