fix: restore consent from ucData when GTM overwrites SDK storage (supabase#44252)

## Problem

After PR supabase#43221 gated `TelemetryTagManager` behind consent, the EU
cookie consent banner started reappearing on every page load and when
navigating between apps (www, studio, docs).

Back in late February we changed `TelemetryTagManager` to only load when
the user has accepted consent. This was the right call for GDPR — don't
load tracking scripts before consent. But it created a chicken-and-egg
problem with how the Usercentrics SDK stores consent.

## What happened

When a user clicks Accept, the SDK writes `uc_settings` +
`uc_user_interaction: true` to localStorage. Then the GTM script loads
(now that consent is granted), and its Usercentrics integration
immediately replaces those keys with a compressed `ucString` + `ucData`
format — deleting the originals.

On the next page load, `UC.init()` only knows how to read `uc_settings`.
It can't find it (GTM deleted it), so it treats the user as brand new
and shows the banner again. Before supabase#43221, GTM loaded on every page
unconditionally, so its integration was already present during
`UC.init()` and could interpret the compressed format.

Confirmed via production console monitoring — the exact sequence after
clicking Accept:

```
setItem("uc_settings", ...)           // SDK writes consent
setItem("uc_user_interaction", "true") // SDK marks interaction
removeItem("uc_settings")             // GTM deletes SDK format
removeItem("uc_user_interaction")     // GTM deletes SDK format
setItem("ucString", ...)              // GTM writes compressed format
setItem("ucData", ...)                // GTM writes compressed format
```

## Changes

- Read `ucData` from localStorage **before** `UC.init()` to detect prior
consent in the compressed format
- If the SDK wants to show the banner but `ucData` shows all services
were previously accepted, silently re-accept instead of re-prompting
- Added try/catch around the SDK initialization (was fire-and-forget
with no error handling, any failure was completely silent)
- Error fallback also honors prior `ucData` consent if the SDK fails to
initialize

## Testing

Can't fully reproduce on staging previews because CSP blocks the GTM
script there (so the storage migration never fires). Verified the root
cause via production console monitoring with localStorage
monkey-patching, and confirmed the `ucData` format persists across page
loads on production.

Closes FE-2648

Author: seanoliver

packages/common/consent-state.ts

@@ -4,6 +4,35 @@ import { proxy, snapshot, useSnapshot } from 'valtio'
 
 import { IS_PLATFORM, LOCAL_STORAGE_KEYS } from './constants'
 
+/**
+ * Check if the user previously accepted all consent services by reading
+ * the compressed ucData format that the GTM/Usercentrics integration writes.
+ *
+ * Context (FE-2648): After acceptAllServices(), the GTM script's Usercentrics
+ * integration replaces uc_settings with ucString/ucData. On the next page load,
+ * UC.init() can't read that format and treats the user as new. This function
+ * detects that prior consent so we can silently re-accept.
+ */
+function hasPreviousConsentInUcData(): boolean {
+  try {
+    const ucData = localStorage?.getItem('ucData')
+    if (!ucData) return false
+
+    const data = JSON.parse(ucData)
+    const services = data?.consent?.services
+    if (!services || typeof services !== 'object') return false
+
+    const serviceValues = Object.values(services)
+    if (serviceValues.length === 0) return false
+
+    return serviceValues.every(
+      (s) => typeof s === 'object' && s !== null && (s as { consent: boolean }).consent === true
+    )
+  } catch {
+    return false
+  }
+}
+
 export const consentState = proxy({
   // Usercentrics state
   UC: null as Usercentrics | null,
@@ -73,27 +102,59 @@ async function initUserCentrics() {
     return
   }
 
-  const { default: Usercentrics } = await import('@usercentrics/cmp-browser-sdk')
-
-  const UC = new Usercentrics(process.env.NEXT_PUBLIC_USERCENTRICS_RULESET_ID!, {
-    rulesetId: process.env.NEXT_PUBLIC_USERCENTRICS_RULESET_ID,
-    useRulesetId: true,
-  })
-
-  const initialUIValues = await UC.init()
-
-  consentState.UC = UC
-  const hasConsented = UC.areAllConsentsAccepted()
-
-  // 0 = first layer, aka show consent toast
-  consentState.showConsentToast = initialUIValues.initialLayer === 0
-  consentState.hasConsented = hasConsented
-  consentState.categories = UC.getCategoriesBaseInfo()
-
-  // If the user has previously consented (before usercentrics), accept all services
-  if (!hasConsented && localStorage?.getItem(LOCAL_STORAGE_KEYS.TELEMETRY_CONSENT) === 'true') {
-    consentState.acceptAll()
-    localStorage.removeItem(LOCAL_STORAGE_KEYS.TELEMETRY_CONSENT)
+  // Check for prior consent BEFORE UC.init(), which can't read the compressed
+  // ucData format written by the GTM/Usercentrics integration (FE-2648).
+  const previouslyAccepted = hasPreviousConsentInUcData()
+
+  try {
+    const { default: Usercentrics } = await import('@usercentrics/cmp-browser-sdk')
+
+    const UC = new Usercentrics(process.env.NEXT_PUBLIC_USERCENTRICS_RULESET_ID!, {
+      rulesetId: process.env.NEXT_PUBLIC_USERCENTRICS_RULESET_ID,
+      useRulesetId: true,
+    })
+
+    const initialUIValues = await UC.init()
+
+    consentState.UC = UC
+    const hasConsented = UC.areAllConsentsAccepted()
+
+    // If the SDK wants to show the banner but the user previously accepted
+    // (ucData exists from a prior GTM-mediated accept), silently re-accept
+    // instead of showing the banner again.
+    if (initialUIValues.initialLayer === 0 && !hasConsented && previouslyAccepted) {
+      consentState.hasConsented = true
+      consentState.showConsentToast = false
+      consentState.categories = UC.getCategoriesBaseInfo()
+      localStorage?.removeItem(LOCAL_STORAGE_KEYS.TELEMETRY_CONSENT)
+      UC.acceptAllServices()
+        .then(() => {
+          consentState.categories = UC.getCategoriesBaseInfo()
+        })
+        .catch(() => {
+          // If re-accept fails, fall back to showing the banner
+          consentState.hasConsented = false
+          consentState.showConsentToast = true
+        })
+      return
+    }
+
+    // 0 = first layer, aka show consent toast
+    consentState.showConsentToast = initialUIValues.initialLayer === 0
+    consentState.hasConsented = hasConsented
+    consentState.categories = UC.getCategoriesBaseInfo()
+
+    // If the user has previously consented (before usercentrics), accept all services
+    if (!hasConsented && localStorage?.getItem(LOCAL_STORAGE_KEYS.TELEMETRY_CONSENT) === 'true') {
+      consentState.acceptAll()
+      localStorage.removeItem(LOCAL_STORAGE_KEYS.TELEMETRY_CONSENT)
+    }
+  } catch (error) {
+    console.error('Failed to initialize Usercentrics:', error)
+    // If SDK fails but user previously accepted, honor that
+    if (previouslyAccepted) {
+      consentState.hasConsented = true
+    }
   }
 }