Offer the search box to whoever can actually search

The sidebar box was drawn for anyone signed in, on the reasoning that
offering it to a signed-out reader gives them a control that can only
answer 401. That stopped being the right test when the guest role became
able to hold assets.view: the box was hidden from exactly the visitor the
grant was reopened for, and a guest could reach search only by typing
/search?q= by hand.

The client cannot decide this. Its hasPermission getter reads the signed-in
user's permission list and a guest has no user at all, so asking it about a
guest answers no for everyone. get_navigation already had the problem and
already solved it by letting the server decide, so capabilities does the
same thing and reports what the CALLER holds.

It reports booleans about the caller and never what the guest role holds in
general, so it tells an anonymous reader nothing the rendered page does not.
Public because it is the pre-login call that decides whether the box is
drawn at all.

Members who lack assets.view now lose a box that only ever 401'd for them.
On a failed call the box stays hidden rather than falling back to the old
signed-in test, which would put it back in front of those same members.

Scope: product

docs/PROJECT-MAP.md

@@ -90,6 +90,6 @@ Manifest-less directories under `plugins/` are core frontend surface and always
 
 ## Size
 
-- test functions defined: **1570** (parametrised cases collect higher)
-- documented API paths: **345** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)
+- test functions defined: **1575** (parametrised cases collect higher)
+- documented API paths: **346** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)
 

docs/api-inventory.json

@@ -423,6 +423,14 @@
         "params": "none",
         "example": "curl http://localhost:5001/api/dashboard/navigation"
       },
+      {
+        "method": "GET",
+        "path": "/api/dashboard/capabilities",
+        "purpose": "Booleans describing what THIS caller may reach, so the app chrome can avoid drawing controls that would only answer 401; cansearch reports whether the caller holds assets.view, which a signed-out visitor does when the guest role has been granted it",
+        "auth": "none",
+        "params": "none",
+        "example": "curl http://localhost:5001/api/dashboard/capabilities"
+      },
       {
         "method": "GET",
         "path": "/api/dashboard/widgets",

docs/openapi.json

@@ -2447,6 +2447,34 @@
     }
    }
   },
+  "/api/dashboard/capabilities": {
+   "get": {
+    "tags": [
+     "core-platform"
+    ],
+    "summary": "Booleans describing what THIS caller may reach, so the app chrome can avoid drawing controls that would only answer...",
+    "description": "Booleans describing what THIS caller may reach, so the app chrome can avoid drawing controls that would only answer 401; cansearch reports whether the caller holds assets.view, which a signed-out visitor does when the guest role has been granted it\n\n**Auth:** none\n\n**Params:** none\n\n**Example:**\n```\ncurl http://localhost:5001/api/dashboard/capabilities\n```",
+    "security": [],
+    "responses": {
+     "200": {
+      "description": "Success. Body is the success_response envelope: {status, data, meta}.",
+      "content": {
+       "application/json": {
+        "$ref": "#/components/schemas/SuccessEnvelope"
+       }
+      }
+     },
+     "default": {
+      "description": "Error. Body is the error envelope; the code and message are nested under data.error.",
+      "content": {
+       "application/json": {
+        "$ref": "#/components/schemas/ErrorEnvelope"
+       }
+      }
+     }
+    }
+   }
+  },
   "/api/dashboard/widgets": {
    "get": {
     "tags": [