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  loops; treat unresolvable remote versions as "potentially outdated"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

backend/src/services/CapabilityRegistry.ts

@@ -1,6 +1,7 @@
 import axios from 'axios';
 import path from 'path';
 import fs from 'fs';
+import semver from 'semver';
 import { SENCHO_VERSION } from '../generated/version';
 
 /**
@@ -35,8 +36,13 @@ export const CAPABILITIES = [
 
 export type Capability = (typeof CAPABILITIES)[number];
 
+/** Returns true when the string is a usable semver version. */
+export function isValidVersion(v: string | null | undefined): v is string {
+  return !!v && v !== 'unknown' && v !== '0.0.0-dev' && !!semver.valid(v);
+}
+
 // Resolved once per process at import time, then cached.
-function resolveVersion(): string {
+function resolveVersion(): string | null {
   if (SENCHO_VERSION !== '0.0.0-dev') return SENCHO_VERSION;
 
   // Fallback for manual ts-node runs without the predev hook.
@@ -49,12 +55,13 @@ function resolveVersion(): string {
     } catch { /* not found, keep walking */ }
     dir = path.dirname(dir);
   }
-  return 'unknown';
+  console.warn('[CapabilityRegistry] Could not resolve Sencho version from any source');
+  return null;
 }
 
 const cachedVersion = resolveVersion();
 
-export function getSenchoVersion(): string {
+export function getSenchoVersion(): string | null {
   return cachedVersion;
 }
 
@@ -83,8 +90,9 @@ export async function fetchRemoteMeta(baseUrl: string, apiToken: string): Promis
       headers: { Authorization: `Bearer ${apiToken}` },
       timeout: 5000,
     });
+    const rawVersion: string | undefined = res.data.version;
     return {
-      version: res.data.version ?? null,
+      version: isValidVersion(rawVersion) ? rawVersion : null,
       capabilities: Array.isArray(res.data.capabilities) ? res.data.capabilities : [],
     };
   } catch (err) {