feat(voice): Assignment aware visibility and join conflict for inbound calls (#14333)

### Description
Inbound voice calls now route ownership cleanly: the call widget is
hidden from agents who aren't the conversation assignee, the first agent
to pick up becomes the assignee, and any later join attempt by another
agent is rejected with a clear "<agent> is already handling the call."
alert.

Closes
https://linear.app/chatwoot/issue/PLA-98/inbound-voice-calls-assignment-aware-visibility-auto-assignment-on

### How to test

1. As Agent A and Agent B, open the dashboard for the same voice inbox
in two browsers.
2. Place an inbound call to the inbox with the conversation
**unassigned** — both agents should see the call widget.
3. Have Agent A click **Join**. Agent A's widget transitions to the
active call; Agent B's widget disappears (conversation is now assigned
to Agent A).
4. While the call is in progress, attempt to join from a third agent
(e.g., via the bubble in the conversation timeline) — the join is
rejected with the toast `Agent A is already handling the call.`
5. Resolve the conversation, then place a second call to a conversation
that is already manually assigned to Agent A — only Agent A sees the
widget; nobody else does.
6. Race test: trigger two near-simultaneous join attempts (two agents
click Join within a few hundred ms of each other) — exactly one wins;
the other gets the conflict alert.

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

app/javascript/dashboard/composables/useCallSession.js

@@ -1,11 +1,14 @@
 import { computed, ref, watch, onUnmounted, onMounted } from 'vue';
+import { useI18n } from 'vue-i18n';
 import VoiceAPI from 'dashboard/api/channel/voice/voiceAPIClient';
 import TwilioVoiceClient from 'dashboard/api/channel/voice/twilioVoiceClient';
 import { useCallsStore } from 'dashboard/stores/calls';
+import { useAlert } from 'dashboard/composables';
 import Timer from 'dashboard/helper/Timer';
 
 export function useCallSession() {
   const callsStore = useCallsStore();
+  const { t } = useI18n();
   const isJoining = ref(false);
   const callDuration = ref(0);
   const durationTimer = new Timer(elapsed => {
@@ -74,6 +77,11 @@ export function useCallSession() {
 
       return { conferenceSid: joinResponse?.conference_sid };
     } catch (error) {
+      useAlert(error?.response?.data?.error || t('CONTACT_PANEL.CALL_FAILED'));
+      if (error?.response?.status === 409) {
+        TwilioVoiceClient.endClientCall();
+        callsStore.dismissCall(callSid);
+      }
       // eslint-disable-next-line no-console
       console.error('Failed to join call:', error);
       return null;