feat: Add unread count filters feature flag (1/6) (#14885)

## Description

Adds the account-level `unread_count_for_filters` feature flag as the
dark-launch gate for filtered sidebar unread counts. This reuses the
deprecated `quoted_email_reply` flag slot, resets the reused bit for
existing accounts, and removes stale defaults so new accounts do not
reference the old flag.

This also adds the feature where we are now calculating the unread counts for built in filters like mentions, participating and unattended along with unread count for saved filters/folders.

Closes
[CW-7262](https://linear.app/chatwoot/issue/CW-7262/unread-counts-for-filters-folders)

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

app/javascript/dashboard/featureFlags.js

@@ -47,6 +47,7 @@ export const FEATURE_FLAGS = {
   ADVANCED_SEARCH: 'advanced_search',
   CONVERSATION_REQUIRED_ATTRIBUTES: 'conversation_required_attributes',
   CONVERSATION_UNREAD_COUNTS: 'conversation_unread_counts',
+  UNREAD_COUNT_FOR_FILTERS: 'unread_count_for_filters',
 };
 
 export const PREMIUM_FEATURES = [