Switch favicon to a black-on-white variant (#557)

## Summary

Regenerate the full favicon set so the Cozystack icon shows as a **black
glyph on a white square**, replacing the previous white-on-`#1a1a1a`
variant.

## Files

All under `static/favicons/`:

- `favicon.svg`, `favicon.ico`, `favicon.png`
- `favicon-16x16.png`, `favicon-32x32.png`
- `apple-touch-icon.png`
- `android-chrome-192x192.png`, `android-chrome-512x512.png`
- `mstile-70x70.png`, `mstile-144x144.png`, `mstile-150x150.png`,
`mstile-310x310.png`
- `mstile-310x150.png` (wide, icon centered on white)

## Notes

- File names and pixel dimensions are unchanged, so `favicons.html`,
`site.webmanifest` and `browserconfig.xml` need no edits.
- The `safari-pinned-tab.svg` mask (and the `<link rel="mask-icon"
color="...">` tint) is intentionally left as-is — it's a monochrome
mask, not a colored icon.
- The site's `theme_color` / `TileColor` (`#1a1a1a`) is left as-is —
those drive browser chrome / tile background, not the icon image itself.

## Test plan

- [ ] Check the browser tab favicon on light and dark themes.
- [ ] Confirm Android/iOS home-screen icons render correctly.

Author: kvaps