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  loops; treat unresolvable remote versions as "potentially outdated"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

frontend/src/lib/version.ts

@@ -0,0 +1,10 @@
+/** Returns true when the string is a displayable version (not a placeholder or missing). */
+export function isValidVersion(v: string | null | undefined): v is string {
+  return !!v && v !== 'unknown' && v !== '0.0.0-dev';
+}
+
+/** Format a version string for display, returning null for invalid/missing values. */
+export function formatVersion(v: string | null | undefined): string | null {
+  if (!isValidVersion(v)) return null;
+  return `v${v}`;
+}