feat(web): Base UI toast region + global mutation feedback wiring (#47)

Add a module-scope Base UI toast manager bridged to the QueryClient so
every mutation can give consistent feedback. A MutationCache (extracted
into a makeQueryClient() factory for test reuse) emits a catch-all,
type-aware error toast (unless meta.suppressErrorToast) and an opt-in
success toast (meta.successMessage), reading mutation.meta + i18n.t
outside React. meta is type-checked via a react-query Register
augmentation. ToastRegion is mounted app-wide in main.tsx. Adds a toast
i18n namespace (en/sv parity) and a validated story.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/i18n/en.json

@@ -59,5 +59,14 @@
     "gateError": "Can't publish — required fields are missing.",
     "editLink": "Edit the record",
     "illegalError": "That visibility change isn't allowed."
+  },
+  "toast": {
+    "created": "Created",
+    "saved": "Saved",
+    "updated": "Updated",
+    "deleted": "Deleted",
+    "renamed": "Renamed",
+    "published": "Visibility updated",
+    "error": "Something went wrong"
   }
 }

web/src/i18n/sv.json

@@ -59,5 +59,14 @@
     "gateError": "Kan inte publicera — obligatoriska fält saknas.",
     "editLink": "Redigera posten",
     "illegalError": "Den synlighetsändringen är inte tillåten."
+  },
+  "toast": {
+    "created": "Skapat",
+    "saved": "Sparat",
+    "updated": "Uppdaterat",
+    "deleted": "Borttaget",
+    "renamed": "Namn ändrat",
+    "published": "Synlighet uppdaterad",
+    "error": "Något gick fel"
   }
 }