feat: expose LLM error reason in /api/ask response and UI

- Add llm_error field to AskResponse so users know why AI summarisation was skipped
- Show orange warning banner in frontend when LLM is not configured or call fails
- Update AskEndpoint tests to assert llm_error presence

backend/frontend/index.html

@@ -53,6 +53,7 @@
       </form>
       <template x-if="askAnswer">
         <div class="ask-result">
+          <div x-show="askLlmError" class="ask-error" x-text="askLlmError"></div>
           <div class="ask-answer" x-html="askAnswerHtml"></div>
           <template x-if="askEvents.length">
             <div class="ask-events">
@@ -245,6 +246,7 @@
         askAnswerHtml: '',
         askEvents: [],
         askLlmUsed: false,
+        askLlmError: '',
 
         async initApp() {
           await this.initAuth();
@@ -501,6 +503,7 @@
             this.askAnswerHtml = this._mdToHtml(body.answer);
             this.askEvents = body.events || [];
             this.askLlmUsed = body.llm_used;
+            this.askLlmError = body.llm_error || '';
           } catch (err) {
             this.askAnswer = 'Sorry, something went wrong: ' + (err.message || 'Unknown error');
             this.askAnswerHtml = this.askAnswer;
@@ -515,6 +518,7 @@
           this.askAnswerHtml = '';
           this.askEvents = [];
           this.askLlmUsed = false;
+          this.askLlmError = '';
         },
 
         _mdToHtml(text) {