feat(settings): link to each provider's key page and clean up base URLs

- A "Get API key" link next to the API Key field for the 19 providers
  that have a key page (from env.example and the providers' docs). 17
  answered 200 to curl; OpenAI's is behind a Cloudflare challenge and
  DeepSeek's behind a regional block, both checked in Chrome
- Base URLs drop spaces, trailing slashes and a pasted endpoint path
  (/chat/completions, /completions, /messages, /responses), which the
  SDK would otherwise append a second time and get a 404. getAIModel does
  this for the chat and the Test button; the field does it on blur and
  shows the URL requests go to

tests/unit/base-url.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from "vitest"
+import { chatRequestUrl, normalizeBaseUrl } from "@/lib/types/model-config"
+
+describe("normalizeBaseUrl", () => {
+    it("drops spaces, trailing slashes and a pasted endpoint path", () => {
+        expect(normalizeBaseUrl(" https://api.x.com/v1/ ")).toBe(
+            "https://api.x.com/v1",
+        )
+        expect(normalizeBaseUrl("https://api.x.com/v1/chat/completions")).toBe(
+            "https://api.x.com/v1",
+        )
+        expect(
+            normalizeBaseUrl("https://api.x.com/anthropic/v1/messages/"),
+        ).toBe("https://api.x.com/anthropic/v1")
+    })
+
+    it("keeps provider paths such as /api/paas/v4 and /api/v3", () => {
+        expect(normalizeBaseUrl("https://open.bigmodel.cn/api/paas/v4")).toBe(
+            "https://open.bigmodel.cn/api/paas/v4",
+        )
+        expect(
+            normalizeBaseUrl("https://ark.cn-beijing.volces.com/api/v3"),
+        ).toBe("https://ark.cn-beijing.volces.com/api/v3")
+    })
+})
+
+describe("chatRequestUrl", () => {
+    it("shows the endpoint the SDK will call", () => {
+        expect(chatRequestUrl("glm", "https://api.x.com/v1/")).toBe(
+            "https://api.x.com/v1/chat/completions",
+        )
+        expect(
+            chatRequestUrl("anthropic", "https://proxy.example.com/v1"),
+        ).toBe("https://proxy.example.com/v1/messages")
+    })
+
+    it("stays out of the way for SDKs that build their own paths", () => {
+        expect(chatRequestUrl("google", "https://x.example.com")).toBeNull()
+        expect(chatRequestUrl("minimax", "https://x.example.com")).toBeNull()
+        expect(chatRequestUrl("glm", "  ")).toBeNull()
+    })
+})