feat: enable linear mode toggle for nightly builds (#8569)

Enables the linear mode toggle for all nightly build users by
short-circuiting the `linearToggleEnabled` feature flag.

## Changes
- Adds `isNightly` check in `linearToggleEnabled` getter  
- Returns `true` for nightly builds, bypassing remote config/server
feature checks
- Adds unit tests for the new behavior

## Reviewers
- @AustinMroz (linear mode maintainer)
- @christian-byrne (isNightly author)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8569-feat-enable-linear-mode-toggle-for-nightly-builds-2fc6d73d3650819681f8dcdc23b6eefe)
by [Unito](https://www.unito.io)


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

## Summary by CodeRabbit

* **New Features**
  * Enabled linear toggle feature for nightly distribution builds.
* Feature flag system now respects nightly vs. standard build
configurations.

* **Tests**
* Added test coverage for nightly build feature flag behavior and remote
configuration handling.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/composables/useFeatureFlags.test.ts

@@ -5,6 +5,7 @@ import {
   ServerFeatureFlag,
   useFeatureFlags
 } from '@/composables/useFeatureFlags'
+import * as distributionTypes from '@/platform/distribution/types'
 import { api } from '@/scripts/api'
 
 // Mock the API module
@@ -14,6 +15,12 @@ vi.mock('@/scripts/api', () => ({
   }
 }))
 
+// Mock the distribution types module
+vi.mock('@/platform/distribution/types', () => ({
+  isCloud: false,
+  isNightly: false
+}))
+
 describe('useFeatureFlags', () => {
   beforeEach(() => {
     vi.clearAllMocks()
@@ -131,4 +138,41 @@ describe('useFeatureFlags', () => {
       expect(maxUploadSize.value).toBe(104857600)
     })
   })
+
+  describe('linearToggleEnabled', () => {
+    it('should return true when isNightly is true', () => {
+      vi.mocked(distributionTypes).isNightly = true
+
+      const { flags } = useFeatureFlags()
+      expect(flags.linearToggleEnabled).toBe(true)
+      expect(api.getServerFeature).not.toHaveBeenCalled()
+    })
+
+    it('should check remote config and server feature when isNightly is false', () => {
+      vi.mocked(distributionTypes).isNightly = false
+      vi.mocked(api.getServerFeature).mockImplementation(
+        (path, defaultValue) => {
+          if (path === ServerFeatureFlag.LINEAR_TOGGLE_ENABLED) return true
+          return defaultValue
+        }
+      )
+
+      const { flags } = useFeatureFlags()
+      expect(flags.linearToggleEnabled).toBe(true)
+      expect(api.getServerFeature).toHaveBeenCalledWith(
+        ServerFeatureFlag.LINEAR_TOGGLE_ENABLED,
+        false
+      )
+    })
+
+    it('should return false when isNightly is false and flag is disabled', () => {
+      vi.mocked(distributionTypes).isNightly = false
+      vi.mocked(api.getServerFeature).mockImplementation(
+        (_path, defaultValue) => defaultValue
+      )
+
+      const { flags } = useFeatureFlags()
+      expect(flags.linearToggleEnabled).toBe(false)
+    })
+  })
 })

src/composables/useFeatureFlags.ts

@@ -1,6 +1,6 @@
 import { computed, reactive, readonly } from 'vue'
 
-import { isCloud } from '@/platform/distribution/types'
+import { isCloud, isNightly } from '@/platform/distribution/types'
 import {
   isAuthenticatedConfigLoaded,
   remoteConfig
@@ -65,6 +65,8 @@ export function useFeatureFlags() {
       )
     },
     get linearToggleEnabled() {
+      if (isNightly) return true
+
       return (
         remoteConfig.value.linear_toggle_enabled ??
         api.getServerFeature(ServerFeatureFlag.LINEAR_TOGGLE_ENABLED, false)