feat: add Free subscription tier support (#8864)

## Summary

Add frontend support for a Free subscription tier — login/signup page
restructuring, telemetry instrumentation, and tier-aware billing gating.

## Changes

- **What**: 
- Restructure login/signup pages: OAuth buttons promoted as primary
sign-in method, email login available via progressive disclosure
- Add Free tier badge on Google sign-up button with dynamic credit count
from remote config
- Add `FREE` subscription tier to type system (tier pricing, tier rank,
registry types)
  - Add `isFreeTier` computed to `useSubscription()`
- Disable credit top-up for Free tier users (dialogService,
purchaseCredits, popover CTA)
- Show subscription/upgrade dialog instead of top-up dialog when Free
tier user hits out-of-credits
- Add funnel telemetry: `trackLoginOpened`, enrich `trackSignupOpened`
with `free_tier_badge_shown`, track email toggle clicks

## Review Focus

- Tier gating logic: Free tier users should see "Upgrade" instead of
"Add Credits" and never reach the top-up flow
- Telemetry event design for Mixpanel funnel analysis
- Progressive disclosure UX on login/signup pages

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8864-feat-add-Free-subscription-tier-support-3076d73d36508133b84ec5f0a67ccb03)
by [Unito](https://www.unito.io)

src/components/topbar/CurrentUserPopoverLegacy.test.ts

@@ -113,12 +113,13 @@ vi.mock('@/platform/cloud/subscription/composables/useSubscription', () => ({
 }))
 
 // Mock the useSubscriptionDialog composable
-const mockSubscriptionDialogShow = vi.fn()
+const mockShowPricingTable = vi.fn()
 vi.mock(
   '@/platform/cloud/subscription/composables/useSubscriptionDialog',
   () => ({
     useSubscriptionDialog: vi.fn(() => ({
-      show: mockSubscriptionDialogShow,
+      show: vi.fn(),
+      showPricingTable: mockShowPricingTable,
       hide: vi.fn()
     }))
   })
@@ -318,8 +319,8 @@ describe('CurrentUserPopoverLegacy', () => {
 
     await plansPricingItem.trigger('click')
 
-    // Verify subscription dialog show was called
-    expect(mockSubscriptionDialogShow).toHaveBeenCalled()
+    // Verify showPricingTable was called
+    expect(mockShowPricingTable).toHaveBeenCalled()
 
     // Verify close event was emitted
     expect(wrapper.emitted('close')).toBeTruthy()

src/components/topbar/CurrentUserPopoverLegacy.vue

@@ -195,7 +195,10 @@ const formattedBalance = computed(() => {
 const canUpgrade = computed(() => {
   const tier = subscriptionTier.value
   return (
-    tier === 'FOUNDERS_EDITION' || tier === 'STANDARD' || tier === 'CREATOR'
+    tier === 'FREE' ||
+    tier === 'FOUNDERS_EDITION' ||
+    tier === 'STANDARD' ||
+    tier === 'CREATOR'
   )
 })
 
@@ -205,7 +208,7 @@ const handleOpenUserSettings = () => {
 }
 
 const handleOpenPlansAndPricing = () => {
-  subscriptionDialog.show()
+  subscriptionDialog.showPricingTable()
   emit('close')
 }