feat(settings): the instance reports which build it is (318 step 6)

A dim line at the foot of Settings: `FabledCurator 2026.08.28.1249 · dev`.

This is no longer a convenience. Milestone 318 stopped publishing version
image tags, so an instance's own report is the ONLY answer to "which build is
this?" — there is no registry name left to check it against. Note #3127 §5
says it directly: a wrong answer here has no second source to contradict it.

Three states, kept distinct because collapsing any two of them lies:

  not asked yet         render nothing
  asked, no version     render "unknown"
  asked, has a version  render it

A blank footer reads as "no version", which is a different claim from "I
cannot say". And a failed health call deliberately does NOT mark the build
loaded — a network blip says nothing about the image, and presenting it as
"unknown" would look like a defective build.

Carried on /api/health rather than a new route: it answers at the same cost
(two module constants, no I/O) and TopNav already fetches it app-wide, so a
separate endpoint would mean a second request for two strings.

Both fields are OMITTED when unset rather than sent empty. Absence already
means "cannot say" — an image predating the field says exactly that by not
having the key — so a second spelling would make every reader special-case
it. The pre-existing test asserting the body is EXACTLY {"status": "ok"} is
what keeps a well-meaning `or ""` default from creeping in.

FC_CHANNEL now has one definition. It was read from the environment in
extension.py and would have been read again here; the new build_info module
holds both, and extension.py binds it as a module-level name so existing
tests monkeypatch it exactly as before. Separate from config.py on purpose:
those are operator settings meant to be changed, these describe the artifact.

Channel sits beside the version, never inside it (rule 149), asserted from
both ends. A `-dev` suffix would read as a 0 segment to the extension's
parseInt comparator and make every dev build compare equal — #2993 exactly.

Not hidden, per the operator and §7: the JS bundle and asset hashes
fingerprint the build anyway, and "I'm on 2026.08.28.1249" is the single most
useful line in a bug report.

frontend/src/stores/system.js

@@ -5,6 +5,18 @@ import { useApi } from '../composables/useApi.js'
 export const useSystemStore = defineStore('system', () => {
   const api = useApi()
   const healthy = ref(null)  // null=unknown, true=ok, false=down
+  // What the instance says it is. Since milestone 318 stopped publishing
+  // version image tags, this is the only answer to "which build is this?" —
+  // there is no registry name left to check it against.
+  //
+  // Three states, and collapsing any two of them would lie:
+  //   buildLoaded=false            we have not asked yet   -> render nothing
+  //   buildLoaded=true, version='' the build cannot say    -> render "unknown"
+  //   buildLoaded=true, version=x  this build is x
+  // A blank footer would read as "no version", which is a different claim.
+  const buildVersion = ref('')
+  const buildChannel = ref('')
+  const buildLoaded = ref(false)
   const stats = ref(null)
   const statsLoading = ref(false)
 
@@ -12,8 +24,17 @@ export const useSystemStore = defineStore('system', () => {
     try {
       const body = await api.get('/api/health')
       healthy.value = body.status === 'ok'
+      // Absent means "cannot say" — the server omits these rather than
+      // sending empty strings, so `?? ''` preserves that rather than
+      // inventing a value for it.
+      buildVersion.value = body.version ?? ''
+      buildChannel.value = body.channel ?? ''
+      buildLoaded.value = true
     } catch {
       healthy.value = false
+      // Deliberately NOT setting buildLoaded: a failed health call tells us
+      // nothing about the build, and claiming "unknown" would present a
+      // network blip as a defective image.
     }
   }
 
@@ -26,5 +47,8 @@ export const useSystemStore = defineStore('system', () => {
     }
   }
 
-  return { healthy, stats, statsLoading, refreshHealth, refreshStats }
+  return {
+    healthy, stats, statsLoading, refreshHealth, refreshStats,
+    buildVersion, buildChannel, buildLoaded,
+  }
 })