fix: merge context + topic into single field — one clear purpose

- Removed separate 'topic' field from BipolarTriggerRequest model
- Removed topic parameter from force_trigger_argument, force_trigger_argument_from_message_id, and run_argument
- trigger_context now doubles as the argument theme: if provided by user, it becomes the topic;
  if blank, a random topic is selected from the rotation pool
- Web UI: replaced two confusing fields (Context + Topic) with one clear field labeled
  'What should they argue about? (optional)' with a plain-English description
- JS: removed topic field reference, context.trim() ensures empty strings aren't sent

bot/static/index.html

@@ -234,15 +234,10 @@
         </div>
         
         <div style="margin-bottom: 1rem;">
-          <label for="bipolar-context">Argument Context (optional):</label>
-          <input type="text" id="bipolar-context" placeholder="e.g., They're fighting about who's the real Miku..." style="width: 100%; margin-top: 0.3rem;">
-        </div>
-        
-        <div style="margin-bottom: 1rem;">
-          <label for="bipolar-topic">Argument Topic (optional — overrides random topic):</label>
-          <input type="text" id="bipolar-topic" placeholder="e.g., Who deserves the spotlight? A philosophical debate about worth..." style="width: 100%; margin-top: 0.3rem;">
+          <label for="bipolar-context">What should they argue about? (optional):</label>
+          <input type="text" id="bipolar-context" placeholder="e.g., Who's the real Miku? Whether kindness is weakness. A petty grudge..." style="width: 100%; margin-top: 0.3rem;">
           <div style="font-size: 0.75rem; color: #777; margin-top: 0.2rem;">
-            Leave blank for random topic selection. Enter a custom theme to frame the argument uniquely.
+            Leave blank for a random topic. Write anything to set the argument's theme.
           </div>
         </div>