fix: hide linear card when not enabled (#12918)

The Linear card now only appears in conversation sidebar when:

- The linear_integration feature flag is enabled for the account
- LINEAR_CLIENT_ID is configured (inferred from the integration existing
in the store)

This matches the backend behavior: if LINEAR_CLIENT_ID is not set, the
integration is filtered out of the API response, so it won't exist in
the store.

In addition, I discovered that Settings/Integrations page showed Linear
card even if it was disabled but Linear client_id set. Now the Linear
card shows only if both conditions are met.

Fixes #12909 

## How Has This Been Tested?

#### Before


https://github.com/user-attachments/assets/cd21b881-5332-48f8-b230-662abc256ba2


#### After



https://github.com/user-attachments/assets/d794cc2e-19d6-4545-b2ef-3af054c2ac81



---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

app/models/integrations/app.rb

@@ -53,7 +53,7 @@ class Integrations::App
     when 'slack'
       GlobalConfigService.load('SLACK_CLIENT_SECRET', nil).present?
     when 'linear'
-      GlobalConfigService.load('LINEAR_CLIENT_ID', nil).present?
+      account.feature_enabled?('linear_integration') && GlobalConfigService.load('LINEAR_CLIENT_ID', nil).present?
     when 'shopify'
       shopify_enabled?(account)
     when 'leadsquared'