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/platform/cloud/subscription/components/SubscriptionPanelContentLegacy.vue

@@ -33,7 +33,7 @@
           </div>
 
           <Button
-            v-if="isActiveSubscription"
+            v-if="isActiveSubscription && !isFreeTier"
             variant="secondary"
             class="ml-auto rounded-lg px-4 py-2 text-sm font-normal text-text-primary bg-interface-menu-component-surface-selected"
             @click="
@@ -213,9 +213,10 @@ import {
   DEFAULT_TIER_KEY,
   TIER_TO_KEY,
   getTierCredits,
-  getTierFeatures,
   getTierPrice
 } from '@/platform/cloud/subscription/constants/tierPricing'
+import type { TierBenefit } from '@/platform/cloud/subscription/utils/tierBenefits'
+import { getCommonTierBenefits } from '@/platform/cloud/subscription/utils/tierBenefits'
 import { cn } from '@/utils/tailwindUtil'
 
 const authActions = useFirebaseAuthActions()
@@ -224,6 +225,7 @@ const { t, n } = useI18n()
 const {
   isActiveSubscription,
   isCancelled,
+  isFreeTier,
   formattedRenewalDate,
   formattedEndDate,
   subscriptionTier,
@@ -264,6 +266,7 @@ const creditsRemainingLabel = computed(() =>
 
 const planTotalCredits = computed(() => {
   const credits = getTierCredits(tierKey.value)
+  if (credits === null) return '—'
   const total = isYearlySubscription.value ? credits * 12 : credits
   return n(total)
 })
@@ -272,48 +275,9 @@ const includedCreditsDisplay = computed(
   () => `${monthlyBonusCredits.value} / ${planTotalCredits.value}`
 )
 
-// Tier benefits for v-for loop
-type BenefitType = 'metric' | 'feature'
-
-interface Benefit {
-  key: string
-  type: BenefitType
-  label: string
-  value?: string
-}
-
-const tierBenefits = computed((): Benefit[] => {
-  const key = tierKey.value
-
-  const benefits: Benefit[] = [
-    {
-      key: 'maxDuration',
-      type: 'metric',
-      value: t(`subscription.maxDuration.${key}`),
-      label: t('subscription.maxDurationLabel')
-    },
-    {
-      key: 'gpu',
-      type: 'feature',
-      label: t('subscription.gpuLabel')
-    },
-    {
-      key: 'addCredits',
-      type: 'feature',
-      label: t('subscription.addCreditsLabel')
-    }
-  ]
-
-  if (getTierFeatures(key).customLoRAs) {
-    benefits.push({
-      key: 'customLoRAs',
-      type: 'feature',
-      label: t('subscription.customLoRAsLabel')
-    })
-  }
-
-  return benefits
-})
+const tierBenefits = computed((): TierBenefit[] =>
+  getCommonTierBenefits(tierKey.value, t, n)
+)
 
 const { totalCredits, monthlyBonusCredits, prepaidCredits, isLoadingBalance } =
   useSubscriptionCredits()