Move heatmap to first-party code (#37262)

Replaces `@silverwind/vue3-calendar-heatmap` with an inlined SVG
implementation. Renders pixel-identically to `main`, drops the
`onMounted` legend viewBox workaround, and uses tippy's
`createSingleton` for the hover tooltip. Adds an e2e test for tooltip
display.

This is a prereq for migrating tippy.js to
[floating-ui](https://github.com/floating-ui/floating-ui) to avoid
having two tooltip libs active.

<img width="861" height="168" alt="image"
src="https://github.com/user-attachments/assets/99343cf6-6e09-42c7-a80d-63dbf33cf56a"
/>


---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>

package.json

@@ -34,7 +34,6 @@
     "@replit/codemirror-lang-svelte": "6.0.0",
     "@replit/codemirror-vscode-keymap": "6.0.2",
     "@resvg/resvg-wasm": "2.6.2",
-    "@silverwind/vue3-calendar-heatmap": "2.1.1",
     "@vitejs/plugin-vue": "6.0.6",
     "ansi_up": "6.0.6",
     "asciinema-player": "3.15.1",