feat: nav counter badges, login backdrop, events i18n + misc fixes

Nav & UI polish
- Sidebar nav items show monospace count badges (projects, sites, stacks,
  proxies). Events badge shows error count only, styled red as actionable
- New $lib/stores/navCounts.ts polls all counts in parallel every 60s and
  refreshes on route change so badges track mutations
- Login page gets a dynamic forge backdrop: rotating conic glow, drifting
  embers, dot-grid texture, vignette — all pure CSS, reduced-motion safe
- main element gets scrollbar-gutter: stable so Settings tab switching no
  longer shifts horizontally when content heights differ

Events i18n
- events.source.* dictionary rewritten to match actually-emitted backend
  sources (deploy, static_site, stale_scanner, stale_cleanup, admin);
  dead keys (container, proxy, system) removed
- EventLogFilter.allSources + /events default sources state updated to match
- Localize "{N} total" via events.totalCount in the page hero toolbar

Backend
- Stage API accepts enable_proxy on create/update (defaults to true) so
  proxy registration can be opted out per stage

Concurrency
- api.ts: queued request waiters no longer double-increment the inflight
  counter; releasing a slot hands it off directly

Reactive effects
- project detail / env / volumes pages wrap side-effect calls in untrack()
  to prevent $effect feedback loops when their loaders mutate tracked state

web/src/lib/i18n/en.json

@@ -817,6 +817,7 @@
     "noEventsDesc": "Events will appear here as they occur.",
     "loadMore": "Load more",
     "newEvents": "new events",
+    "totalCount": "{count} total",
     "clearAll": "Clear All",
     "clearAllTitle": "Clear Event Log",
     "clearAllMessage": "This will permanently delete all event log entries. This cannot be undone.",
@@ -840,9 +841,10 @@
     },
     "source": {
       "deploy": "Deploy",
-      "container": "Container",
-      "proxy": "Proxy",
-      "system": "System"
+      "static_site": "Static Site",
+      "stale_scanner": "Stale Scanner",
+      "stale_cleanup": "Stale Cleanup",
+      "admin": "Admin"
     },
     "metadata": "Details"
   },