feat: add optional custom argument topic override via Web UI

- Added optional 'topic' field to BipolarTriggerRequest model
- Added topic parameter to force_trigger_argument and force_trigger_argument_from_message_id
- Updated run_argument to accept optional custom topic (None=random, ''=no topic, str=custom)
- Added topic input field to Web UI trigger-argument section
- Updated JS to send topic in API request body
- Custom topics bypass the random rotation system, allowing manual theme control

bot/static/index.html

@@ -238,6 +238,14 @@
           <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;">
+          <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.
+          </div>
+        </div>
+        
         <button onclick="triggerBipolarArgument()" style="background: #9932CC; color: #fff; border: none; padding: 0.5rem 1rem; border-radius: 4px; cursor: pointer;">
           ⚔️ Trigger Argument
         </button>