chore: Strip unsupported signature formatting by channel (#13046)

# Pull Request Template

## Description

1. This PR is an enhancement to
https://github.com/chatwoot/chatwoot/pull/13045
It strips unsupported formatting from **message signatures** based on
each channel’s formatting capabilities defined in the `FORMATTING`
config

2. Remove usage of plain editor in Compose new conversation modal

Only the following signature elements are considered:
<strong>bold (<code inline="">strong</code>), italic (<code
inline="">em</code>), links (<code inline="">link</code>), images (<code
inline="">image</code>)</strong>.</p>

Any formatting not supported by the target channel is automatically
removed before the signature is appended.

<h3>Channel-wise Signature Formatting Support</h3>

Channel | Keeps in Signature | Strips from Signature
-- | -- | --
Email | bold, italic, links, images | —
WebWidget | bold, italic, links, images | —
API | bold, italic | links, images
WhatsApp | bold, italic | links, images
Telegram | bold, italic, links | images
Facebook | bold, italic | links, images
Instagram | bold, italic | links, images
Line | bold, italic | links, images
SMS | — | everything
Twilio SMS | — | everything
Twitter/X | — | everything


<hr>
<h3>📝 Note</h3>
<blockquote>
<p>Message signatures only support <strong>bold, italic, links, and
images</strong>.<br>
Other formatting options available in the editor (lists, code blocks,
strike-through, etc.) do <strong>not apply</strong> to signatures and
are ignored.</p>
</blockquote>

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Loom video
https://www.loom.com/share/d325ab86ca514c6d8f90dfe72a8928dd


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/components-next/NewConversation/components/ComposeNewConversationForm.vue

@@ -6,6 +6,7 @@ import { INBOX_TYPES } from 'dashboard/helper/inbox';
 import {
   appendSignature,
   removeSignature,
+  getEffectiveChannelType,
 } from 'dashboard/helper/editorHelper';
 import {
   buildContactableInboxesList,
@@ -86,6 +87,12 @@ const whatsappMessageTemplates = computed(() =>
 
 const inboxChannelType = computed(() => props.targetInbox?.channelType || '');
 
+const inboxMedium = computed(() => props.targetInbox?.medium || '');
+
+const effectiveChannelType = computed(() =>
+  getEffectiveChannelType(inboxChannelType.value, inboxMedium.value)
+);
+
 const validationRules = computed(() => ({
   selectedContact: { required },
   targetInbox: { required },
@@ -202,7 +209,11 @@ const removeSignatureFromMessage = () => {
   // Always remove the signature from message content when inbox/contact is removed
   // to ensure no leftover signature content remains
   if (props.messageSignature) {
-    state.message = removeSignature(state.message, props.messageSignature);
+    state.message = removeSignature(
+      state.message,
+      props.messageSignature,
+      effectiveChannelType.value
+    );
   }
 };
 
@@ -214,9 +225,9 @@ const removeTargetInbox = value => {
 };
 
 const clearSelectedContact = () => {
+  removeSignatureFromMessage();
   emit('clearSelectedContact');
   state.attachedFiles = [];
-  removeSignatureFromMessage();
 };
 
 const onClickInsertEmoji = emoji => {
@@ -227,12 +238,16 @@ const handleAddSignature = signature => {
   state.message = appendSignature(
     state.message,
     signature,
-    inboxChannelType.value
+    effectiveChannelType.value
   );
 };
 
 const handleRemoveSignature = signature => {
-  state.message = removeSignature(state.message, signature);
+  state.message = removeSignature(
+    state.message,
+    signature,
+    effectiveChannelType.value
+  );
 };
 
 const handleAttachFile = files => {
@@ -356,10 +371,10 @@ const shouldShowMessageEditor = computed(() => {
       v-model="state.message"
       :message-signature="messageSignature"
       :send-with-signature="sendWithSignature"
-      :is-email-or-web-widget-inbox="inboxTypes.isEmailOrWebWidget"
       :has-errors="validationStates.isMessageInvalid"
       :has-attachments="state.attachedFiles.length > 0"
       :channel-type="inboxChannelType"
+      :medium="targetInbox?.medium || ''"
     />
 
     <AttachmentPreviews