fix: go build module cache permission + README sudo

- Set GOMODCACHE + GOPATH to /opt/receiptnext/.go (writable location)
- Removes sudo from README one-liner — script self-elevates
- Build now works for users whose home directory isn't writable

README.md

@@ -9,7 +9,7 @@
 ## 🚀 One-Command Install
 
 ```bash
-sudo bash -c "$(curl -fsSL https://git.lohmar.co.uk/cclohmar/ReceiptNext/raw/branch/main/install.sh)"
+bash -c "$(curl -fsSL https://git.lohmar.co.uk/cclohmar/ReceiptNext/raw/branch/main/install.sh)"
 ```
 
 Or from a local clone:

install.sh

@@ -126,6 +126,10 @@ esac
 
 if command -v go &>/dev/null; then
     info "Building binary from source (pure Go, no CGO)..."
+    # Use a writable module cache — the user's home may not be writable
+    export GOMODCACHE="${INSTALL_DIR}/.go/mod"
+    export GOPATH="${INSTALL_DIR}/.go"
+    mkdir -p "${GOMODCACHE}" 2>/dev/null || sudo_if mkdir -p "${GOMODCACHE}"
     CGO_ENABLED=0 go build -ldflags="-s -w" -o app .
 else
     info "Go not found — downloading pre-built binary..."