feat: show dev-<sha> version string in nav for dev builds

Production images continue to display the semver (v1.x.x). Dev images
built by CI now receive BUILD_VERSION=dev-<7-char-sha> via a Docker ARG,
and app.js skips the v prefix for non-semver strings.

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

Dockerfile

@@ -7,8 +7,13 @@ COPY package*.json ./
 RUN npm ci --omit=dev
 
 COPY . .
-RUN awk -F'"' '/"version"/{printf "const VERSION = \"%s\";\n", $4; exit}' \
-    package.json > js/version.js
+ARG BUILD_VERSION=""
+RUN if [ -n "$BUILD_VERSION" ]; then \
+      printf 'const VERSION = "%s";\n' "$BUILD_VERSION" > js/version.js; \
+    else \
+      awk -F'"' '/"version"/{printf "const VERSION = \"%s\";\n", $4; exit}' \
+          package.json > js/version.js; \
+    fi
 
 RUN mkdir -p /app/data && chown -R app:app /app
 USER app