Disable send button when required player not selected in quick chat (#767)

## Description:
This PR prevents players from sending Quick Chat messages that require a
player target ([P1]) unless a player has been selected. The Send button
is now disabled in such cases and visually dimmed for clarity.
## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:
aotumuri

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- The chat send button is now disabled if a required player selection is
missing, preventing unintended sends.

- **Style**
- The chat send button visually indicates when it is disabled with a
gray background and a not-allowed cursor.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/client/graphics/layers/ChatModal.ts

@@ -173,7 +173,8 @@ export class ChatModal extends LitElement {
           <button
             class="chat-send-button"
             @click=${this.sendChatMessage}
-            ?disabled=${!this.previewText}
+            ?disabled=${!this.previewText ||
+            (this.requiresPlayerSelection && !this.selectedPlayer)}
           >
             ${translateText("chat.send")}
           </button>

src/client/styles/modal/chat.css

@@ -57,6 +57,11 @@
   cursor: pointer;
 }
 
+.chat-send-button:disabled {
+  background: #666;
+  cursor: not-allowed;
+}
+
 .chat-column {
   display: flex;
   flex-direction: column;