fix: quote wallet build output path

[manuelsampedro1]

This hardens the wallet build script so the configured distribution folder is handled safely.

Changes:

• fail early with a clear usage message when no dist folder is provided
• quote every dist-folder path expansion used by mkdir/cp/babel/uglify/html-minifier/cleancss/rm
• preserve the existing build flow and generated filenames

This prevents confusing shell failures when the output path is empty or contains spaces.

Validation:

• bash -n build.sh
• ./build.sh without arguments returns the new usage error before invoking npm
• stubbed build-tool smoke test against /tmp/bitgesell wallet dist test confirmed quoted paths with spaces reach the expected outputs

Bounty context: BitgesellOfficial/bitgesell#39

[MyTH-zyxeon]

Maintainer review note for the #81/#39 bounty queue:

I checked head be2ff3e: this is a focused build-script portability fix, not a wallet runtime/key-handling change. The useful behavior is quoting the dist-folder path across mkdir, cp, Babel, UglifyJS, rm, HTML minification, and CleanCSS, while adding an explicit missing-argument guard before running the build.

Acceptance checks:

• Run the existing build script with a dist folder containing spaces, for example ./build.sh "dist test", and confirm app.min.js, index.html, and custom.min.css land under that exact directory.
• Confirm the new empty-argument guard exits before npm test, so an accidental missing dist path fails fast without writing to the repo root.
• Verify the generated assets are byte-equivalent or intentionally minifier-equivalent for a normal dist path without spaces.
• Keep this scoped to build artifact path handling only; no wallet seed/key generation, storage, transaction, or runtime behavior should change.

No wallet/key/live-chain scope; this is build-path hygiene only.