feat: Heatmaps UI

[benjackwhite]

Problem

Draft approach at https://github.com/PostHog/posthog/issues/21531

Changes

• [x] Adds a flagged beta UI for heatmaps
• [x] Allows searching existing URLs to load

TODO

• [ ] Only allow loading authorized URLs (and give a way to change that easily here)
• [ ] Add controls to the main UI so that we can sync over iframe messages
• [ ] Detect if the toolbar and show a message if not and how to troubleshoot (sending them to the toolbar instead)

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

How did you test this code?

[posthog-bot]

📸 UI snapshots have been updated

4 snapshot changes in total. 0 added, 4 modified, 0 deleted:

• chromium: 0 added, 4 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

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[github-actions[bot]]

Size Change: +784 B (0%)

Total Size: 1.05 MB

ℹ️ View Unchanged

Filename	Size	Change
frontend/dist/toolbar.js	1.05 MB	+784 B (0%)

_{compressed-size-action}

[posthog-bot]

📸 UI snapshots have been updated

2 snapshot changes in total. 0 added, 2 modified, 0 deleted:

• chromium: 0 added, 2 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

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[posthog-bot]

📸 UI snapshots have been updated

1 snapshot changes in total. 0 added, 1 modified, 0 deleted:

• chromium: 0 added, 1 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

Triggered by this commit.

👉 Review this PR's diff of snapshots.

[posthog-bot]

📸 UI snapshots have been updated

1 snapshot changes in total. 0 added, 1 modified, 0 deleted:

• chromium: 0 added, 1 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

Triggered by this commit.

👉 Review this PR's diff of snapshots.