fix: draw the API key once, and let status answer the dimensions question

Two things a user setting this up hit immediately.

The card drew `embeddingApiKey` twice. The render loop walked every entry in
FIELDS, and the secret — which is drawn by hand, as a write-only password
control — was one of them, so a second control appeared beside it: an ordinary
text input, echoing the key in clear as you typed. One key, two boxes, one of
them visible. The loop skips secrets now, and the tests count controls rather
than checking that each expected label is present, because a duplicate beside
the real control passed the old assertions.

`status` could not answer the one question a new user cannot look up: how wide
is this model? The probe that reports it was gated on every required setting
being present, so in exactly the state that needs the answer — dimensions
unset — it reported nothing and said "Missing: embedding_dimensions". It now
runs whenever the endpoint can be named (a model and a base URL), and the
status message names the width it observed:

  Missing: embedding_dimensions. The endpoint is reachable and returned 768
  dimensions, so set embeddingDimensions to 768.

The probe also tells "not set yet" apart from "set wrong", because the two have
different remedies.

Also documents both ways to read the width by hand, since a wrong value is
frozen into the index at first use and costs a full re-embed to correct.

lib/probe.js

@@ -156,7 +156,12 @@ export async function probeEmbeddingEndpoint({
     // is not lossless JSON and fails the tool's output validation before the
     // result is ever materialised.
     if (actual !== expected) {
-      result.error = `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`
+      // "not set yet" and "set wrong" are different problems with different
+      // remedies, and `dimensions: actual` is the answer to the first one.
+      result.error =
+        expected > 0
+          ? `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`
+          : `the model returns ${actual} dimensions and embeddingDimensions is not set`
     }
     return result
   } catch (error) {