fix: say the true thing when Gmail, AI or mail is not set up

Two bugs with one shape: the interface confidently told you something
that was not so.

Connecting Gmail without a registered OAuth client said "Det er der
allerede." Thirteen different situations answered a plain `conflict`, and
the frontend renders that as "that already exists" — which for four of
them is not vague but wrong. An unconfigured Gmail is not a Gmail that is
already connected; a second factor that is off is not one that is on; the
Inbox is not a project that already exists somewhere else. Those four
have their own codes now, and their own sentences.

The mail-to-task address came out as todo+…@localhost. VERDANDE_SMTP_FROM
defaults to verdande@localhost and the domain was taken from it
unconditionally, so an instance with no mail server handed out an address
that looks real, can never receive anything, and gives no hint which of
the two it is. The default no longer wins over the public hostname, and
the response now carries whether a mail server exists at all — because
even with the right domain, nothing routes to it until one does.

errors.go has always claimed the code list is kept in one place "so the
frontend's mapping can be checked against it". Nothing checked it, and
the mapping was missing bad_request. Now a test reads both files and
fails on a code with no message.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/api.md

@@ -52,6 +52,10 @@ what a person reads is decided by the client, which knows what language they are
 | `forbidden` | 403 | Cross-site request, or admin needed. |
 | `not_found` | 404 | Does not exist, or you cannot see it. |
 | `conflict` | 409 | Already exists, or the state does not allow it. |
+| `gmail_not_configured` | 409 | No Google OAuth client is registered on this server. |
+| `ai_not_configured` | 409 | No AI provider has been set up. |
+| `totp_not_enabled` | 409 | Two-factor is off, and the request needs it on. |
+| `inbox_protected` | 409 | The Inbox cannot be deleted. |
 | `validation_failed` | 422 | Check `fields`. |
 | `rate_limited` | 429 | Too many attempts. |
 | `internal_error` | 500 | Our fault. |

docs/openapi.yaml

@@ -1946,6 +1946,13 @@ components:
             type: object
             properties:
               address: { type: string, example: todo+abc123@example.dk }
+              configured:
+                type: boolean
+                description: |
+                  Whether a mail server is set up at all. The address is real
+                  either way, but with no mail server nothing on the internet
+                  routes to it — and an interface that does not say so is
+                  promising something that will not happen.
     Gmail:
       description: OK
       content:
@@ -1995,6 +2002,10 @@ components:
             - rate_limited
             - internal_error
             - payload_too_large
+            - gmail_not_configured
+            - ai_not_configured
+            - totp_not_enabled
+            - inbox_protected
         error: { type: string }
         fields:
           type: object