fix frontend to display backend error messages to users

the backend was correctly returning helpful error messages (like "search query is too long..."), but the frontend was only showing "search failed: Bad Request" because it only read response.statusText and never read the response body.

now when an API error occurs, the frontend:
1. reads the response body text
2. displays the actual error message from the backend
3. falls back to statusText if body reading fails

this means users will now see the full helpful message: "search query is too long (max 1024 characters for text search). try a shorter query."

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

static/index.html

···
                 });
 
                 if (!response.ok) {
-                    throw new Error(`search failed: ${response.statusText}`);
+                    // try to extract error message from response body
+                    let errorMessage = response.statusText;
+                    try {
+                        const errorText = await response.text();
+                        // actix-web returns plain text error messages, not JSON
+                        if (errorText) {
+                            errorMessage = errorText;
+                        }
+                    } catch (e) {
+                        // if reading body fails, use the status text
+                    }
+                    throw new Error(errorMessage);
                 }
 
                 const data = await response.json();