feat(provisioning): admin game-signup mode setting, invite link option, staff self-create

Follow-ups from live testing:

- Game-account creation is now an admin Settings control (disabled / website /
  hybrid / game) instead of a hidden on/off flag. The site offers creation for
  website+hybrid; help text notes the shard's SignupMode (Bridge.cfg) has the final
  say. game_account_signup setting widened to a 4-value enum + validated on save.
- Invites: the accept link is ALWAYS returned and shown with a Copy button, and a
  "Email the invitation" toggle lets an admin create a link-only invite (no email)
  or email it. Backend takes sendEmail (default true) and always returns acceptUrl.
- Staff can create a game account from their own /admin/characters page too
  (POST /admin/shard/account → the shared createGameAccount controller), so the
  form is reachable in both the player and admin portals.

Note: the admin Houses view (/admin/houses) already worked; the earlier failure
was a stale Vite HMR state for the new route (needs a hard refresh).

Client build clean; server routes load; swagger regenerated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/swagger/swagger-output.json

@@ -2426,6 +2426,63 @@
         ]
       }
     },
+    "/api/v1/admin/shard/account": {
+      "post": {
+        "tags": [
+          "Admin · Account"
+        ],
+        "summary": "Create a game account and link it to the caller (staff self-service)",
+        "description": "Same as POST /player/shard/account but for a signed-in staff user — provisions a game account (own username + password) and links it. Gated by game_account_signup + the shard’s mode; the password is never stored or logged.",
+        "responses": {
+          "201": {
+            "description": "Account created and linked",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "additionalProperties": true
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Bad Request"
+          },
+          "403": {
+            "description": "Game-account signup unavailable (site or shard)",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
+          "409": {
+            "description": "Account name already taken",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
+          "500": {
+            "description": "Internal Server Error"
+          }
+        },
+        "security": [
+          {
+            "cookieAuth": []
+          },
+          {
+            "bearerAuth": []
+          }
+        ],
+        "requestBody": {}
+      }
+    },
     "/api/v1/admin/shard/kick": {
       "post": {
         "tags": [