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  loops; treat unresolvable remote versions as "potentially outdated"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/components/EditorLayout.tsx

@@ -26,6 +26,7 @@ import { LabelPill, LabelDot, type Label as StackLabel } from './LabelPill';
 import { LabelAssignPopover } from './LabelAssignPopover';
 import { UserProfileDropdown } from './UserProfileDropdown';
 import { apiFetch, fetchForNode } from '@/lib/api';
+import { isValidVersion } from '@/lib/version';
 import { toast } from '@/components/ui/toast-store';
 import { Label } from './ui/label';
 import { Command, CommandInput, CommandList, CommandItem } from './ui/command';
@@ -1350,7 +1351,7 @@ export default function EditorLayout() {
                           node.status === 'offline' ? 'bg-red-500' : 'bg-gray-400'
                           }`} />
                         <span>{node.name}</span>
-                        {meta?.version && meta.version !== 'unknown' && meta.version !== '0.0.0-dev' && (
+                        {isValidVersion(meta?.version) && (
                           <span className="font-mono text-[10px] tabular-nums text-muted-foreground/60 ml-auto">
                             v{meta.version}
                           </span>