fix(www): skip PostHog when api key is missing

[Yeom-JinHo]

Description

This PR makes PostHog optional in the www app when NEXT_PUBLIC_POSTHOG_API_KEY is not configured.
It prevents local contributor environments from hitting PostHog initialization errors while preserving analytics behavior when a key is present.
It also routes analytics events through a shared helper so missing-key environments fail gracefully.

Changes

• Added a shared PostHog utility to centralize enablement checks, one-time initialization, and guarded event capture.
• Updated the PostHog provider to skip initialization and provider wrapping when the API key is missing.
• Replaced direct posthog.capture(...) calls in banner and sidebar CTA components with the shared event path.
• Extended the tracked event schema to include the affected CTA and banner events.
• Added rel="noopener noreferrer" to the external links touched by this change.

Motivation

Contributors running the app without a local PostHog key currently hit analytics-related errors while navigating the site.
This change improves the local development experience by making analytics opt-in based on environment configuration instead of assuming the key is always available.

Breaking Changes

None.

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