fix: upgrade to Node.js 20 for Vite 7 compatibility

- CI workflow: NODE_VERSION 18 → 20
- Dockerfile: node:18-alpine → node:20-alpine (all stages)

Vite 7 requires Node.js 20.19+ or 22.12+

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 env:
   PYTHON_VERSION: "3.11"
-  NODE_VERSION: "18"
+  NODE_VERSION: "20"
 
 jobs:
   test-backend:

frontend/Dockerfile

@@ -4,7 +4,7 @@
 # ==============================================================================
 
 # Stage 1: Dependencies
-FROM node:18-alpine AS deps
+FROM node:20-alpine AS deps
 
 WORKDIR /app
 
@@ -15,7 +15,7 @@ COPY package.json package-lock.json ./
 RUN npm ci
 
 # Stage 2: Builder
-FROM node:18-alpine AS builder
+FROM node:20-alpine AS builder
 
 WORKDIR /app
 
@@ -27,7 +27,7 @@ COPY . .
 RUN npm run build
 
 # Stage 3: Production
-FROM node:18-alpine AS production
+FROM node:20-alpine AS production
 
 WORKDIR /app