fix(fleet): resolve version detection pipeline for Docker builds (#402)

* fix(fleet): resolve version detection pipeline for Docker builds

The Dockerfile backend-builder stage was missing a COPY of the root
package.json, causing generate-version.js to fall back to "0.0.0-dev"
at build time. At runtime, the filesystem walk also failed (root
package.json not in the final image), producing the string "unknown"
which the frontend rendered as "vunknown".

Changes:
- Dockerfile: copy root package.json into backend-builder stage
- CapabilityRegistry: return null (not "unknown") for unresolvable
  versions; add isValidVersion() type guard; normalize remote meta
  responses to strip "unknown"/"0.0.0-dev" sentinel values
- Fleet endpoints: hoist gateway version validation outside per-node
  loops; treat unresolvable remote versions as "potentially outdated"
  instead of silently marking them up to date
- FleetView: guard all version display points (card badge, update
  button, gateway label, modal columns) via shared formatVersion()
- EditorLayout, CapabilityGate: use shared isValidVersion utility
- New frontend/src/lib/version.ts shared utility
- Docs: add troubleshooting section for version display edge cases
- Screenshots: updated Fleet Overview and Node Updates modal

* docs: update fleet node updates screenshot with live remote node

Dockerfile

@@ -33,6 +33,8 @@ RUN npm config set fetch-retry-maxtimeout 120000 && \
     npm install
 
 COPY backend/ ./
+# prebuild hook (generate-version.js) reads the root package.json for the app version
+COPY package.json /app/package.json
 RUN npm run build
 
 # Stage 3: Production dependencies (cross-compiled - NO QEMU execution)