fix: add npm ci retry and install verification for proxy environments (#9)

npm has a known bug where `npm ci` can crash with "Exit handler never
called!" behind corporate proxies yet exit with code 0. This adds a
single retry on failure and verifies tsc is actually installed before
proceeding to build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -6,7 +6,9 @@ FROM node:20-alpine AS frontend
 WORKDIR /app/web
 
 COPY web/ .
-RUN npm ci --include=dev && npm run build
+RUN npm ci --include=dev || npm ci --include=dev && \
+    node_modules/.bin/tsc --version && \
+    npm run build
 
 # Stage 2: Build Go binary
 FROM golang:1.25-alpine AS builder