Move PostHog gating into `buildFeatureChecks`

[n-lark]

Move PostHog gating into buildFeatureChecks

Problem

PostHog gating is hand-wired into the featuresCheck getter in account-settings.js, separate from the config-driven feature system in FeatureChecks.ts. The pairing between a PostHog flag and its platform feature is implicit.

Goal

Let a feature config declare an optional posthogKey alongside platformKey/teamKey. PostHog is authoritative: when it's available, its flag value drives the check regardless of platform/team settings. Only when PostHog is unavailable does the check fall back to the existing platform/team logic.

Behavior

For a config with a posthogKey, buildFeatureChecks resolves {output} as:

window.posthog present	Has platform/team key	Result
yes	—	PostHog flag value
no	yes	existing platform/team check
no	no	false

Configs with no posthogKey are unchanged.

Changes

• Add posthogKey?: string to FeatureConfig; apply the table above in buildFeatureChecks. Availability is !!window.posthog; the value comes from posthogFlags (passed in; still populated by loadPosthogFlags → onFeatureFlags so the getter stays reactive).
• Move POSTHOG_FLAGS and the three PostHog lines out of the featuresCheck getter in account-settings.js (keep posthogFlags state + loadPosthogFlags).
• Reconcile the MCP output name: ExpertButton.vue reads isMcpThirdPartyEnabled, config output is isMcpThirdPartyFeatureEnabled.
• Remove isPostHogFeatureFlagsEnabled entirely, plus the code it gates in General.vue (the v-if Expert-agent creds template block and the mounted-hook assignment). Drop the FF_FEATURE_FLAGS key.

This changes MCP: today it's platform AND team AND PostHog; under the new rule PostHog trumps everything when present, so its value drives the check regardless of platform/team.