Feature Request(heatmaps): support custom screenshot uploads

Feature request

Is your feature request related to a problem?

Heatmaps today only work against pages that PostHog can reach by URL. The three current rendering modes (iframe, screenshot, recording) all assume either a live web page that loads in an iframe, a URL that PostHog can crawl, or a session replay HTML snapshot. For users building desktop or native applications (for example, an Electron app), there is no reachable URL to target heatmaps, so the feature is effectively unavailable.

Describe the solution you'd like

Add a fourth heatmap type that accepts a user-uploaded image as the background. The user uploads a screenshot of their app screen, PostHog stores it like the existing per-width screenshot snapshots in HeatmapSnapshot.content, and the existing click overlay renders on top.

Describe alternatives you've considered

• The toolbar capture flow. Only available in browser contexts.

Additional context

• Customer use case: a desktop application similar to Electron that wants to understand interaction patterns the same way web teams use heatmaps today.
• Raised in: https://posthoghelp.zendesk.com/agent/tickets/57875 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/58897)

Debug info

Start by tracing the existing heatmap rendering modes and the HeatmapSnapshot.content storage path mentioned in the issue. Compare how iframe, screenshot, and recording heatmaps are created and rendered; done means a user-uploaded image is stored as a heatmap snapshot and the existing click overlay renders on it.