ci: build the web app with Trunk and deploy it to GitHub Pages from main

pages.yml builds apps/fieldcraft-web (trunk build --release, wasm-opt)
on pushes to main and develop and on manual runs, and deploys the site
to https://jyviko.github.io/fieldcraft/ only from main (releases).
Develop merges that touch code build without deploying, so a broken
web build shows up before a release. Trunk is a pinned release binary
checked against its SHA-256; actions are pinned by commit.

Part of #119

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

ATTRIBUTION.md

@@ -177,6 +177,7 @@ is committed and listed here with its exports.
 | packaging/windows/check-exe.ps1 | SolidCraft `packaging/windows/check-icon.ps1` | MIT OR Apache-2.0 |
 | apps/fieldcraft-web/ (the web runner, browser file services and downloads, Trunk configuration, index.html) | VectorCraft `apps/vectorcraft-web/` and SolidCraft `apps/solidcraft-web/` | MIT OR Apache-2.0 |
 | crates/io/src/memory.rs (files kept in memory for the browser build) | SolidCraft `crates/io/src/vfs.rs` | MIT OR Apache-2.0 |
+| .github/workflows/pages.yml | SolidCraft `.github/workflows/pages.yml` | MIT OR Apache-2.0 |
 | .github/workflows/release.yml, .github/release-notes.md, docs/releasing.md | SolidCraft `.github/workflows/release.yml`, `.github/release-notes.md`, `docs/releasing.md`; VectorCraft `.github/workflows/release.yml` | MIT OR Apache-2.0 |
 | .github/workflows/packaging-lint.yml | SolidCraft `.github/workflows/packaging-lint.yml` (itself adapted from PhotoCraft, https://github.com/storytold/photocraft, Apache-2.0) | MIT OR Apache-2.0 |
 | .github/workflows/arch-check.yml (the riscv64 cross build) | SolidCraft `.github/workflows/ci.yml` (`riscv64-gates`) | MIT OR Apache-2.0 |