Rework the first-run setup wizard around what a site cannot run without

The wizard configured five of the product's 129 settings. A site that finished
it still had no timezone, no authentication, no mail server, no backup
destination and no endpoint agent policy, and nothing told the operator so.

Six steps now: Administrator, Site, Authentication, Features, Mail and Backups,
Review. Everything else moves to a standing setup checklist at
/settings/setupchecklist, because a wizard that asked for all 129 settings would
be worse than one that asked for none.

Two new endpoints back it. /setup/checklist reports twelve areas as
ok/partial/todo/na with a link to the page that fixes each, and /setup/health
reports account counts plus whether a plugin toggle has reached every worker.
Both take care not to lie:

- Several settings are SEEDED non-empty (the shipped logo, a placeholder floor
  plan, every identifier field), so "is it set" compares against the shipped
  default rather than emptiness.
- plugin.endpoints.sharedkeyallowed DEFAULTS TO ALLOWED when its row is absent,
  so an absent row is reported as the fleet-wide key being accepted. Reporting
  it as refused told an admin the opposite of the truth about the credential
  they are migrating away from.
- A plugin is held at "starting" until several DISTINCT workers report it live.
  Counting replies instead let one recycled worker answer four times and declare
  a change complete while other workers still served the old routes.

Recommended plugins are declared in each plugin's manifest rather than listed in
the wizard, so adding a plugin cannot leave a stale list in another file.

Also fixed: the logo preview was invisible, because the image borrowed a class
that hardcodes a white background while the shipped logo is white; and signing
in did not re-read setup state, so an admin already browsing was never steered
to the wizard and a hard refresh was the only cure.

(cherry picked from commit 32d3f4b29d)

plugins/knowledgebase/manifest.json

@@ -3,11 +3,14 @@
   "display_name": "Knowledge Base",
   "version": "1.0.0",
   "description": "Knowledge Base articles linking to external resources",
+  "recommended": true,
   "author": "ShopDB Team",
   "dependencies": [],
   "core_version": ">=0.1.0,<1.0.0",
   "api_prefix": "/api/knowledgebase",
   "provides": {
-    "features": ["knowledgebase"]
+    "features": [
+      "knowledgebase"
+    ]
   }
 }