diff --git a/.gitignore b/.gitignore
index ab309a3..c93bc46 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,8 +3,5 @@ __pycache__/
 *.log
 *.log.[0-9]
 /dist/
-
-# Claude Code local files
-.claude/
-scripts/
+# Scratch config created by scripts/run-dev-ui
 .devdata/
diff --git a/scripts/run-dev-ui.ps1 b/scripts/run-dev-ui.ps1
new file mode 100644
index 0000000..255b2d3
--- /dev/null
+++ b/scripts/run-dev-ui.ps1
@@ -0,0 +1,76 @@
+# run-dev-ui.ps1 - start the multiACE web UI locally, against a real
+# Moonraker if one is reachable and against tests\fixtures\*.json if not.
+#
+#   .\scripts\run-dev-ui.ps1
+#   $env:MOONRAKER_URL = "http://printer:7125"; .\scripts\run-dev-ui.ps1
+#   $env:MULTIACE_MOCK_MODE = "1"; .\scripts\run-dev-ui.ps1   # force mock
+#   $env:MULTIACE_MOCK_STATE_FILE = "mock_state_head.json"; .\scripts\run-dev-ui.ps1
+#       # dashboard in head mode instead of the default multi-ACE fixture
+#
+# Mock mode also unlocks POST /api/debug/simulate (inject a fake error
+# notification) and GET /api/debug/sample-gcode (a small 4-filament
+# sample print, tests\fixtures\sample_4color.gcode).
+
+$ErrorActionPreference = "Stop"
+
+$Port = if ($env:PORT) { $env:PORT } else { "7126" }
+$MoonrakerUrl = if ($env:MOONRAKER_URL) { $env:MOONRAKER_URL } else { "http://127.0.0.1:7125" }
+$RepoRoot = Split-Path -Parent $PSScriptRoot
+$BackendDir = Join-Path $RepoRoot "multiace\web\backend"
+
+if ($env:MULTIACE_MOCK_MODE) {
+    Write-Host "-> mock mode forced by MULTIACE_MOCK_MODE=$($env:MULTIACE_MOCK_MODE)"
+} else {
+    try {
+        Invoke-WebRequest -Uri "$MoonrakerUrl/printer/info" -TimeoutSec 2 -UseBasicParsing | Out-Null
+        Write-Host "OK  Moonraker found at $MoonrakerUrl - using real data"
+        $env:MULTIACE_MOCK_MODE = "0"
+    } catch {
+        Write-Host "--  Moonraker not reachable at $MoonrakerUrl - using mock data"
+        $env:MULTIACE_MOCK_MODE = "1"
+    }
+}
+
+$env:MOONRAKER_URL = $MoonrakerUrl
+if (-not $env:MULTIACE_MOCK_DIR) {
+    $env:MULTIACE_MOCK_DIR = Join-Path $RepoRoot "tests\fixtures"
+}
+# The installer copies the catalogs next to the backend (web\i18n); in a
+# checkout they live at multiace\i18n. Without this every label in the UI
+# renders as its raw key.
+if (-not $env:MULTIACE_I18N_DIR) {
+    $env:MULTIACE_I18N_DIR = Join-Path $RepoRoot "multiace\i18n"
+}
+# A dev box has no /home/lava; point the config editor at a scratch copy
+# so saving from the UI cannot 404 (and cannot touch a real printer).
+if (-not $env:MULTIACE_CFG_PATH) {
+    $devCfg = Join-Path $RepoRoot ".devdata\ace.cfg"
+    $devDir = Split-Path -Parent $devCfg
+    if (-not (Test-Path $devDir)) { New-Item -ItemType Directory -Path $devDir | Out-Null }
+    if (-not (Test-Path $devCfg)) {
+        Copy-Item (Join-Path $RepoRoot "multiace\config\extended\ace.cfg") $devCfg
+    }
+    $env:MULTIACE_CFG_PATH = $devCfg
+}
+Write-Host "    config: $($env:MULTIACE_CFG_PATH)"
+
+$Python = if ($env:PYTHON) { $env:PYTHON } else { "python" }
+$Url = "http://127.0.0.1:$Port/"
+
+Start-Job -ScriptBlock {
+    param($u)
+    Start-Sleep -Seconds 2
+    Start-Process $u
+} -ArgumentList $Url | Out-Null
+
+if ($env:MULTIACE_MOCK_MODE -eq "1") {
+    Write-Host "    mock: sample print at http://127.0.0.1:$Port/api/debug/sample-gcode"
+}
+
+Write-Host "Dev server on $Url  (Ctrl+C to stop)"
+Push-Location $BackendDir
+try {
+    & $Python -m uvicorn main:app --host 127.0.0.1 --port $Port --reload
+} finally {
+    Pop-Location
+}
diff --git a/scripts/run-dev-ui.sh b/scripts/run-dev-ui.sh
new file mode 100755
index 0000000..2376073
--- /dev/null
+++ b/scripts/run-dev-ui.sh
@@ -0,0 +1,84 @@
+#!/bin/bash
+# run-dev-ui.sh - start the multiACE web UI locally, against a real
+# Moonraker if one is reachable and against tests/fixtures/*.json if not.
+#
+#   ./scripts/run-dev-ui.sh                 # auto-detect
+#   MOONRAKER_URL=http://printer:7125 ./scripts/run-dev-ui.sh
+#   MULTIACE_MOCK_MODE=1 ./scripts/run-dev-ui.sh   # force mock
+#   MULTIACE_MOCK_STATE_FILE=mock_state_head.json ./scripts/run-dev-ui.sh
+#       # dashboard in head mode instead of the default multi-ACE fixture
+#
+# Mock mode also unlocks POST /api/debug/simulate (inject a fake error
+# notification) and GET /api/debug/sample-gcode (a small 4-filament
+# sample print, tests/fixtures/sample_4color.gcode).
+set -e
+
+PORT="${PORT:-7126}"
+MOONRAKER_URL="${MOONRAKER_URL:-http://127.0.0.1:7125}"
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BACKEND_DIR="$REPO_ROOT/multiace/web/backend"
+
+# Git Bash / MSYS hands POSIX paths ("/c/Users/…") to a NATIVE Windows
+# Python, which reads them as drive-relative and silently finds nothing -
+# every label in the UI then renders as its raw i18n key. Convert every
+# path we export. No-op on Linux/macOS, where cygpath does not exist.
+winpath() {
+    if command -v cygpath >/dev/null 2>&1; then cygpath -w "$1"; else printf '%s' "$1"; fi
+}
+
+if [ -n "$MULTIACE_MOCK_MODE" ]; then
+    echo "-> mock mode forced by MULTIACE_MOCK_MODE=$MULTIACE_MOCK_MODE"
+elif curl -fsS --max-time 2 "$MOONRAKER_URL/printer/info" >/dev/null 2>&1; then
+    echo "OK  Moonraker found at $MOONRAKER_URL - using real data"
+    MULTIACE_MOCK_MODE=0
+else
+    echo "--  Moonraker not reachable at $MOONRAKER_URL - using mock data"
+    MULTIACE_MOCK_MODE=1
+fi
+
+export MULTIACE_MOCK_MODE
+export MOONRAKER_URL
+export MULTIACE_MOCK_DIR="${MULTIACE_MOCK_DIR:-$(winpath "$REPO_ROOT/tests/fixtures")}"
+# The installer copies the catalogs next to the backend (web/i18n); in a
+# checkout they live at multiace/i18n. Without this every label in the UI
+# renders as its raw key.
+export MULTIACE_I18N_DIR="${MULTIACE_I18N_DIR:-$(winpath "$REPO_ROOT/multiace/i18n")}"
+# A dev box has no /home/lava; point the config editor at a scratch copy
+# so saving from the UI cannot 404 (and cannot touch a real printer).
+if [ -z "$MULTIACE_CFG_PATH" ]; then
+    MULTIACE_CFG_PATH="$REPO_ROOT/.devdata/ace.cfg"
+    mkdir -p "$(dirname "$MULTIACE_CFG_PATH")"
+    [ -f "$MULTIACE_CFG_PATH" ] || cp "$REPO_ROOT/multiace/config/extended/ace.cfg" "$MULTIACE_CFG_PATH"
+    MULTIACE_CFG_PATH="$(winpath "$MULTIACE_CFG_PATH")"
+    export MULTIACE_CFG_PATH
+fi
+echo "  config: $MULTIACE_CFG_PATH"
+
+# Pick an interpreter that actually RUNS: a python3 that merely exists on
+# PATH is not enough (a broken Windows Store / stale install shim is on
+# most dev boxes, and it fails only once uvicorn is already "started").
+PY=""
+for cand in "$PYTHON" python3 python py; do
+    [ -n "$cand" ] || continue
+    if "$cand" -c 'import sys' >/dev/null 2>&1; then PY="$cand"; break; fi
+done
+if [ -z "$PY" ]; then
+    echo "No working Python found - set PYTHON=/path/to/python" >&2
+    exit 1
+fi
+
+URL="http://127.0.0.1:$PORT/"
+(
+    sleep 2
+    if command -v xdg-open >/dev/null 2>&1; then xdg-open "$URL"
+    elif command -v open >/dev/null 2>&1; then open "$URL"
+    fi
+) &
+
+if [ "$MULTIACE_MOCK_MODE" = "1" ]; then
+    echo "  mock: sample print at http://127.0.0.1:$PORT/api/debug/sample-gcode"
+fi
+
+echo "Dev server on $URL  (Ctrl+C to stop)"
+cd "$BACKEND_DIR"
+exec "$PY" -m uvicorn main:app --host 127.0.0.1 --port "$PORT" --reload