feat: validate credentials via /auth_health before saving

Call `GET <url>/auth_health` with `Authorization` and `x-tenant` headers
after the user completes the 3-step login wizard. Only persist credentials
to `~/.altimate/altimate.json` if the server returns `{"status": "auth_valid"}`.
Shows inline error on the URL step if validation fails.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: suryaiyer95

packages/opencode/src/cli/cmd/tui/component/dialog-altimate-login.tsx

@@ -1,4 +1,4 @@
-import { createSignal } from "solid-js"
+import { createSignal, Show } from "solid-js"
 import { useDialog } from "@tui/ui/dialog"
 import { useSDK } from "../context/sdk"
 import { useSync } from "@tui/context/sync"
@@ -17,8 +17,38 @@ export function DialogAltimateLogin() {
   const [step, setStep] = createSignal<"instance" | "key" | "url">("instance")
   const [instanceName, setInstanceName] = createSignal("")
   const [apiKey, setApiKey] = createSignal("")
+  const [error, setError] = createSignal("")
+  const [validating, setValidating] = createSignal(false)
+
+  async function validateAndSave(url: string) {
+    setError("")
+    setValidating(true)
+    try {
+      const res = await fetch(`${url}/auth_health`, {
+        method: "GET",
+        headers: {
+          Authorization: `Bearer ${apiKey()}`,
+          "x-tenant": instanceName(),
+        },
+      })
+      if (!res.ok) {
+        setError("Invalid credentials — check your instance name, API key, and URL")
+        setValidating(false)
+        return
+      }
+      const data = await res.json()
+      if (data.status !== "auth_valid") {
+        setError("Unexpected response from server")
+        setValidating(false)
+        return
+      }
+    } catch (e) {
+      setError(`Connection failed — could not reach ${url}`)
+      setValidating(false)
+      return
+    }
+    setValidating(false)
 
-  async function saveAndConnect(url: string) {
     const creds = {
       altimateUrl: url,
       altimateInstanceName: instanceName(),
@@ -67,11 +97,19 @@ export function DialogAltimateLogin() {
           placeholder="https://api.myaltimate.com"
           value="https://api.myaltimate.com"
           description={() => (
-            <text fg={theme.textMuted}>Enter your Altimate server URL</text>
+            <box gap={1}>
+              <text fg={theme.textMuted}>Enter your Altimate server URL</text>
+              <Show when={validating()}>
+                <text fg={theme.textMuted}>Validating credentials...</text>
+              </Show>
+              <Show when={error()}>
+                <text fg={theme.error}>{error()}</text>
+              </Show>
+            </box>
           )}
           onConfirm={async (value) => {
             if (!value) return
-            await saveAndConnect(value)
+            await validateAndSave(value)
           }}
         />
       )}