feat: explicit provider field replaces URL sniffing

LlmConfig.provider ("ollama" | "openai", empty = sniff URL so legacy
configs keep working). Settings presets set it — a custom-port Ollama
no longer falls into the OpenAI branch and fails confusingly.

Also: test_connection now accepts an optional config override — the
wizard was passing one that Rust silently ignored, so it tested the
saved config instead of the URL the user just typed.

src-tauri/src/llm/mod.rs

@@ -25,6 +25,12 @@ pub struct AppState {
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct LlmConfig {
+    /// API dialect: "ollama" (native /api/*) or "openai" (/v1/*).
+    /// Empty = sniff the URL (legacy configs + defaults). Provider presets
+    /// set it explicitly — a custom-port Ollama would otherwise be sniffed
+    /// as OpenAI and fail confusingly. serde default: old stored configs.
+    #[serde(default)]
+    pub provider: String,
     pub api_url: String,
     pub api_key: String,
     pub model: String,
@@ -42,6 +48,7 @@ impl Default for LlmConfig {
     fn default() -> Self {
         Self {
             // Default to Ollama local server; also works with LM Studio, llama.cpp server, or OpenAI
+            provider: String::new(),
             api_url: "http://localhost:11434".to_string(),
             api_key: String::new(),
             model: "llama3.2".to_string(),
@@ -100,9 +107,18 @@ pub struct OllamaChatResponse {
     pub done: bool,
 }
 
-// ─── Helper: detect if we're talking to Ollama ───────────────
+// ─── Helper: are we talking Ollama? ───────────────────────
 
-pub fn is_ollama(url: &str) -> bool {
+/// True when we should speak Ollama's native /api/* protocol. An explicit
+/// provider on the config wins; empty falls back to URL sniffing so legacy
+/// and default configs keep working.
+pub fn is_ollama(provider: &str, url: &str) -> bool {
+    if provider.eq_ignore_ascii_case("ollama") {
+        return true;
+    }
+    if provider.eq_ignore_ascii_case("openai") {
+        return false;
+    }
     url.contains("localhost:11434") || url.contains("127.0.0.1:11434")
 }