fix(server): use the keys the user sent, and more review fixes

Found by the second PR review:
- With AWS_BEARER_TOKEN_BEDROCK set on the server, a request with the
  user's AWS keys ran on the server's token: the Bedrock SDK prefers it.
  Checked with Bedrock: invalid user keys used to get an answer.
- An OpenAI key with the official URL filled in (the settings form does
  that) went to the Responses API. Back to main's rule: a configured base
  URL uses Chat Completions.
- A user's Ollama key went to the server's OLLAMA_BASE_URL, for chat and
  for the model list. Like every other provider, it goes to the user's
  base URL or Ollama Cloud.
- The server's keyless Ollama and EdgeOne were not counted in the quota.
- AI_MODEL models on the server's keys ran on any provider with a server
  key, not only on AI_PROVIDER.
- A user's Azure key without a base URL used the server's resource name.
- The admin panel's Test button failed whenever access codes were set.
- DeepSeek's errors in the stream (plain text) were shown as they were,
  without a hint and also on the server's keys. Bedrock's throttling in
  the stream was not recognised as a rate limit.
- The EdgeOne function accepted text/plain; x=application/json, which
  other sites can send without a CORS preflight.
- Desktop app: a launch that found the old port taken for a moment (the
  previous version still quitting after an update) remembered the new
  port for good. The new port is kept only when Windows reserves the old
  one. A failed read of the presets file moved it aside as corrupt, and a
  save could then replace the presets. Switching presets on the same port
  now reloads the page. The dev launcher no longer misses a preset change
  made before or during a restart.

tests/unit/llm-errors.test.ts

@@ -229,6 +229,27 @@ describe("streamErrorText", () => {
             message: "bad key",
         })
     })
+
+    it("classifies a provider error sent as plain text", () => {
+        // DeepSeek's SDK sends errors in the stream as a string
+        const text = "Insufficient Balance for account 42"
+        expect(JSON.parse(streamErrorText(text))).toEqual({
+            type: "provider",
+            code: "insufficient_quota",
+            message: text,
+        })
+        expect(JSON.parse(streamErrorText(text, true)).message).not.toMatch(
+            /account 42/,
+        )
+    })
+
+    it("classifies Bedrock's throttling sent in the stream", () => {
+        // Bedrock's ThrottlingException as a plain object, not an API error
+        const throttled = {
+            message: "Too many tokens, please wait before trying again.",
+        }
+        expect(JSON.parse(streamErrorText(throttled)).code).toBe("rate_limited")
+    })
 })
 
 describe("isToolCallError", () => {