fix: privacy strip — zero third-party calls from the desktop app (stream C)

[gitjehan]

Summary

The imported desktop code phoned home in several places. This PR strips all of it — the app binary now talks to exactly one endpoint: our own API proxy (APIBaseURL in Info.plist).

• C1 — analytics + email capture removed: ClickyAnalytics.swift deleted along with all 16 call sites, the PostHog SDK package, and the onboarding email form that posted addresses to submit-form.com.
• C2 — external media removed: the mux.com-hosted intro video is gone; onboarding now runs a live pointing demo on the user's own screen instead (arguably a better demo). The x.com social button is deleted.
• C3 — direct-to-vendor clients deleted: ElementLocationDetector (direct api.anthropic.com calls), the OpenAI chat + transcription clients, and the AssemblyAI provider (dead upstream worker URL). The STT factory is now hardwired to Apple's on-device recognizer. The unused Sparkle updater and the upstream developer's appcast feed are also gone.

Net: −1,880 lines, two fewer package dependencies.

Verification

• Headless xcodebuild → BUILD SUCCEEDED
• Static URL audit of all Swift sources + Info.plist → only http://localhost (APIBaseURL) remains

What's left

• C4 runtime check: watch Console/proxy logs during the first full GUI run (B6) to confirm the only network destination is our proxy.

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