feat(config): allow placeholder text to be string or list

Allow PlaceholderConfig.Text to accept either a single string or an
array of strings, from which one is randomly selected at runtime.
This maintains backward compatibility with existing single-string configs
while enabling random placeholder selection.

Changes:
- Modify PlaceholderConfig.Text type from string to FlexibleStringSlice
- Add GetRandomText() helper method for random selection
- Update SendPlaceholder in all channels to use GetRandomText()
- Update config.example.json with array placeholder examples
- Update Matrix channel documentation

config/config.example.json

@@ -129,6 +129,10 @@
       "encrypt_key": "",
       "verification_token": "",
       "allow_from": [],
+      "placeholder": {
+        "enabled": true,
+        "text": ["Thinking...", "Processing...", "Typing..."]
+      },
       "reasoning_channel_id": "",
       "random_reaction_emoji": [],
       "is_lark": false
@@ -160,7 +164,7 @@
       },
       "placeholder": {
         "enabled": true,
-        "text": "Thinking... 💭"
+        "text": ["Thinking...", "Processing...", "Typing..."]
       },
       "reasoning_channel_id": "",
       "crypto_database_path": "",