fix: show human-readable login errors instead of raw HTML

When the server returns an HTML response (wrong URL, redirect to a
login page), the error dialog previously showed the full HTML body.
Now detect HTML responses and show a short actionable message:
- 404 with HTML: 'API endpoint not found. Check your server URL.'
- 401/403 with HTML: 'Wrong username or password.'
- 200 with HTML (no Auth= token): explain the endpoint is not FreshRSS
- Non-HTML bodies are shown as-is (they are already readable)

src/api.rs

@@ -73,13 +73,26 @@ impl Api {
         let body = resp.text().await.map_err(|e| e.to_string())?;
 
         if !status.is_success() {
-            return Err(format!("Login failed ({}): {}", status, body.trim()));
+            let msg = human_error(&body, status.as_u16());
+            return Err(format!("Login failed ({}): {}", status.as_u16(), msg));
         }
 
         let auth_token = body
             .lines()
             .find_map(|l| l.strip_prefix("Auth="))
-            .ok_or_else(|| "No Auth token in response".to_string())?
+            .ok_or_else(|| {
+                // The server returned 200 but not the expected API response —
+                // most likely the URL points to a web page, not a FreshRSS API.
+                if looks_like_html(&body) {
+                    format!(
+                        "The server at {} does not appear to be a FreshRSS \
+                         Google Reader API endpoint. Check your server URL.",
+                        base
+                    )
+                } else {
+                    format!("Unexpected response from server: {}", body.trim())
+                }
+            })?
             .to_string();
 
         Ok(Self {
@@ -195,6 +208,28 @@ impl Api {
     }
 }
 
+fn looks_like_html(body: &str) -> bool {
+    let trimmed = body.trim_start();
+    trimmed.starts_with("<!") || trimmed.to_ascii_lowercase().starts_with("<html")
+}
+
+fn human_error(body: &str, status: u16) -> String {
+    if looks_like_html(body) {
+        match status {
+            401 | 403 => "Wrong username or password.".to_string(),
+            404 => "API endpoint not found. Check your server URL.".to_string(),
+            _ => format!("Server returned HTTP {status}. Check your server URL."),
+        }
+    } else {
+        let trimmed = body.trim();
+        if trimmed.is_empty() {
+            format!("Server returned HTTP {status} with no message.")
+        } else {
+            trimmed.to_string()
+        }
+    }
+}
+
 fn plain_text_excerpt(html: &str, max_chars: usize) -> String {
     // Very simple HTML stripper — remove tags, collapse whitespace
     let mut out = String::with_capacity(html.len());