Fix production build: npm registry flakiness, stray package-lock.json dir, TS error

- registry.npmjs.org was intermittently ETIMEDOUT mid-download from this
  host; switched to registry.npmmirror.com (also raised npm's own retry
  budget as a second line of defense).
- frontend/package-lock.json on disk was actually an empty directory
  (untracked, root-owned — a stray artifact, likely from an old bad
  bind-mount), which broke `COPY . .` once npm install got past it.
- documents/page.tsx had a real type error (selectedItem.file narrowing
  didn't survive into the .map() closure) that `next dev` never
  surfaced but `next build`'s stricter check does — fixed by hoisting
  the narrowed array into selectedFiles once instead of re-narrowing
  selectedItem.file at each use.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

frontend/Dockerfile

@@ -9,7 +9,14 @@ WORKDIR /app
 ENV NODE_OPTIONS=--dns-result-order=ipv4first
 
 COPY package.json package-lock.json* ./
-RUN npm install
+# registry.npmjs.org itself is unreliable from this host (mid-download
+# stalls even with aggressive npm retries) — route through npmmirror.com
+# instead, which is consistently reachable from this network.
+RUN npm config set registry https://registry.npmmirror.com && \
+    npm config set fetch-retries 8 && \
+    npm config set fetch-retry-mintimeout 20000 && \
+    npm config set fetch-retry-maxtimeout 120000 && \
+    npm install
 
 COPY . .
 RUN npm run build