Favicon doesn't match assetmantle.one branding

## What

The favicon served at https://explorer.assetmantle.one/assets/images/Favicon.ico is the legacy explorer icon (a non-AssetMantle mark). It doesn't match the website (`assetmantle.one`) which uses its own pixel-style AssetMantle favicon.

After PR #102 + #103 aligned the explorer's logo, fonts, palette and lightMode defaults with the website, the favicon is the last visible brand mismatch — a tab from the explorer doesn't read as AssetMantle in browser tab strips, bookmarks bars, or pinned tabs.

## What it should be

The same favicon set that ships with the website repo:

- Source: `~/github.com/AssetMantle/website/app/favicon.ico` (Next.js convention)
- Likely we want both `.ico` (back-compat) and a higher-res PNG / SVG plus `apple-touch-icon` and a `manifest.webmanifest` for PWA-style installs.

## Suggested fix

1. Copy `favicon.ico` from `AssetMantle/website` into `AssetMantle/client/public/images/Favicon.ico` (replacing the current file).
2. Optionally add `favicon-32.png`, `favicon-192.png`, `apple-touch-icon.png` and update `` in `app/views/base/common.scala.html` to reference them.
3. While there: the existing `` is unfingerprinted; switch it to `@routes.Assets.versioned("images/Favicon.ico")` so the new `sbt-digest` pipeline (PR #101) auto-invalidates the cache when the icon changes.

## Acceptance

- New tab on `https://explorer.assetmantle.one/` shows the same icon as `https://assetmantle.one/`.
- Pinned/bookmarked tabs in Chrome/Safari/Firefox show the AssetMantle pixel mark.
- Cache-Control on the icon URL is `max-age=31536000, immutable` (because it's now a fingerprinted asset).

## Context (for future picker)

- Explorer is on Akash DSEQ 26587209 (provider `akash1x2g8wfa429fukudgkclaag00d00z4rn846j7wq`); current image `ghcr.io/assetmantle/client:lightmode-lift-v4` digest `sha256:6d9e0e36`.
- Repo-root pickup notes: `THREAD_PICKUP_2026-04-29.md`.
- Style alignment work that motivated this issue: PR #102 (logo/fonts/palette/light/GoL) and PR #103 (light-mode design lift).

🤖 Filed via Claude Code

Compare website/app/favicon.ico with client/public/images/Favicon.ico, then inspect app/views/base/common.scala.html and the asset versioning setup from PR #101. Confirm the intended favicon formats and update the references so the explorer uses the AssetMantle mark, fingerprinted asset URLs, and the stated cache behavior.

Written by the indexing model from the issue text.