change cloud feature flags to be loaded dynamically at runtime rather than set in build (#6246)

## Summary

Implements server-side remote configuration to decouple runtime behavior
from build artifacts, enabling dynamic configuration updates without
redeployment.

## Technical Changes

- **Replaced** build-time constants (`__MIXPANEL_TOKEN__`,
`__BUILD_FLAGS__`) with runtime configuration loaded from
`/api/features`
- Configuration now sourced from `window.__CONFIG__` (hydrated from
`/api/features` endpoint)
- **Added** `src/platform/remoteConfig/` service that polls server
configuration every 30 seconds
- **Modified** application bootstrap sequence in `main.ts` to load
remote config before module initialization (required for cloud builds)
- **Removed** global constants: `__BUILD_FLAGS__`, `__MIXPANEL_TOKEN__`.
Runtime subscription enforcement toggle via `subscription_required` flag
- Server health alerts with variant-based severity rendering
(info/warning/error) via topbar badges

## Rationale

- **Build-once-deploy-anywhere**: Single immutable artifact promoted
through environments (staging → production)
- **Zero-downtime configuration**: Update behavior without rebuilding or
redeploying the application
- **Incident response**: Disable features or display alerts dynamically
in response to outages or degraded service
- **Instant rollback**: Revert configuration changes server-side without
artifact redeployment
- **Progressive delivery**: Enable A/B testing, canary releases, and
user/region-based configuration
- **Environment parity**: Eliminate configuration drift between staging
and production builds
- Decouples deployment cadence from configuration changes
- Enables GitOps workflows for configuration management separate from
code deployments
- Supports real-time operational control of client behavior
- Reduces build matrix complexity (no per-environment builds)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6246-change-cloud-feature-flags-to-be-loaded-dynamically-at-runtime-rather-than-set-in-build-2966d73d3650811cbb41c9093961037a)
by [Unito](https://www.unito.io)

src/platform/settings/composables/useSettingUI.ts

@@ -81,7 +81,7 @@ export function useSettingUI(
   }
 
   const subscriptionPanel: SettingPanelItem | null =
-    !isCloud || !__BUILD_FLAGS__.REQUIRE_SUBSCRIPTION
+    !isCloud || !window.__CONFIG__?.subscription_required
       ? null
       : {
           node: {
@@ -149,7 +149,9 @@ export function useSettingUI(
       keybindingPanel,
       extensionPanel,
       ...(isElectron() ? [serverConfigPanel] : []),
-      ...(isCloud && __BUILD_FLAGS__.REQUIRE_SUBSCRIPTION && subscriptionPanel
+      ...(isCloud &&
+      window.__CONFIG__?.subscription_required &&
+      subscriptionPanel
         ? [subscriptionPanel]
         : [])
     ].filter((panel) => panel.component)
@@ -185,12 +187,12 @@ export function useSettingUI(
         userPanel.node,
         ...(isLoggedIn.value &&
         isCloud &&
-        __BUILD_FLAGS__.REQUIRE_SUBSCRIPTION &&
+        window.__CONFIG__?.subscription_required &&
         subscriptionPanel
           ? [subscriptionPanel.node]
           : []),
         ...(isLoggedIn.value &&
-        !(isCloud && __BUILD_FLAGS__.REQUIRE_SUBSCRIPTION)
+        !(isCloud && window.__CONFIG__?.subscription_required)
           ? [creditsPanel.node]
           : [])
       ].map(translateCategory)