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>

tests/e2e/heatmap.test.ts

@@ -0,0 +1,9 @@
+import {test, expect} from '@playwright/test';
+import {login} from './utils.ts';
+
+test('heatmap tooltip shows on hover', async ({page}) => {
+  await login(page);
+  await page.goto('/');
+  await page.locator('.heatmap-day').first().hover();
+  await expect(page.locator('.tippy-box[data-state="visible"]')).toBeVisible();
+});