fix: danmaku template now uses event rules; rules apply instantly on save

- _on_danmaku renders via event_text(rules, 'danmaku', ...) like all other
  events; hardcoded {uname}说、 {msg} format removed
- Drop message_format param from DanmakuHandler/DanmakuClient/server/main
- api_rules_post mutates the shared rules dict in place (clear+update)
  instead of rebinding, so running handlers pick up new templates
  immediately without restarting TTS
- UI save toast: '规则已保存并即时生效'
- Verified: custom template 卧槽！、是{uname}！、他说、 {msg} works;
  save-then-next-danmaku uses new template without restart

static/index.html

@@ -634,7 +634,7 @@ async function saveRules() {
     const data = await resp.json();
     if (data.error) { alert('保存失败: ' + data.error); return; }
     renderRules(data.rules);
-    alert('规则已保存。重启TTS后生效。');
+    alert('规则已保存并即时生效。');
   } catch(e) {
     alert('保存失败: ' + e.message);
   }