v0.112.0: self-update without credentials — anonymous Docker v2 token flow

Root cause (live on Peti's box): the updater piggybacked on Git Sync creds and refused when absent,
but the registry serves the public package anonymously (verified 2026-07-10). Credentials become what
they were meant to be — optional, for private catalogs only.

- queryRegistry: both creds empty → anonymous flow (plain GET → parse WWW-Authenticate realm/service
  from the header, never hardcoded → credential-free token → Bearer retry); creds present → BasicAuth
  path unchanged; half-configured pair → loud incomplete-credentials error
- pullImage: no creds → skip docker login entirely (docker's native anonymous flow); denied anonymous
  access → clear 'registry denied anonymous access — a private registry requires Git Sync credentials'
- settings page: 'Registry: nyilvános (hitelesítés nélkül) / hitelesített' mode line — credential-less
  is no longer an error state; DryRun.PullCapable counts anonymous as capable
- tests: fake registry httptest token dance (zero creds, no auth on token request, correct scope),
  creds path unchanged (BasicAuth, no dance), both denial paths, WWW-Authenticate parser table
  (quoted/bare/order/comma-in-quotes/missing-realm), fake-runner pull tests (no login invoked
  anonymously; login/pull/logout order + stdin token with creds; partial creds refuse)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

controller/internal/web/templates/settings_system.html

@@ -87,6 +87,10 @@
             <span class="settings-value mono">{{.LastCheckTime}}</span>
         </div>
         {{end}}
+        <div class="settings-row">
+            <span class="settings-label">Registry</span>
+            <span class="settings-value">{{if .RegistryAnonymous}}nyilvános (hitelesítés nélkül){{else}}hitelesített{{end}}</span>
+        </div>
         {{if .LastCheckError}}
         <div class="settings-row">
             <span class="settings-label">Hiba</span>