fix(host,web): an empty update channel stops looking like a broken host

A channel nobody has published to answers `manifest.json` with a 404, and the
check reported that the same way it reports a dead registry or a bad signature:
"Last check failed: feed returned HTTP 404". Every host on the stable channel
shows it today, because the stable manifest only publishes when someone
dispatches `announce` for a release tag — so the first thing an operator sees
from the new Updates card is a red failure caused by nothing being wrong.

The shared checker now distinguishes the two. `feed::fetch_manifest_blocking`
returns a typed `FeedError` instead of a string, and only a 404 on the manifest
ITSELF becomes `NotPublished` — a 404 on the detached signature still fails
loudly, because that is the half-published pair the manifest-then-signature
upload order can produce, and it must stay fail-closed.

The host carries that through as `UpdateStatus.not_published`, mutually
exclusive with `last_error`. It is benign only while no manifest has ever been
seen for the channel: once a check has succeeded, the same 404 means the feed
LOST a document it used to serve, which stays an error. The console then shows a
plain sentence naming the channel instead of the failure banner, and "None
published yet" rather than "Not checked yet".

The Linux client makes the same distinction but deliberately NOT the same
choice: `--check-update` keeps exiting 1 and keeps `error` set, because its
consumer is a shell script and an empty channel is the absence of evidence that
this build is current — not a confirmation that it is.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

api/openapi.json

@@ -3495,7 +3495,7 @@
         "operationId": "forceUpdateCheck",
         "responses": {
           "200": {
-            "description": "Refreshed update-check state (`last_error` carries a failed check)",
+            "description": "Refreshed update-check state (`last_error` carries a failed check; `not_published` an empty channel, which is not one)",
             "content": {
               "application/json": {
                 "schema": {
@@ -7372,7 +7372,8 @@
           "apply",
           "channel_hint",
           "check_disabled",
-          "available"
+          "available",
+          "not_published"
         ],
         "properties": {
           "apply": {
@@ -7452,6 +7453,10 @@
               }
             ]
           },
+          "not_published": {
+            "type": "boolean",
+            "description": "The check reached the feed and found this channel has **no release published yet** —\nan expected state (a channel nobody has announced to answers with a 404), not a\nfailure. Mutually exclusive with `last_error`, so a UI can say \"nothing published yet\"\ninstead of painting an empty feed as a broken host. Never set once a manifest has been\nseen for this channel: a feed that loses a document it used to serve stays an error."
+          },
           "opt_in_hint": {
             "type": [
               "string",