docs(install): document the web UI build dependencies - #1651

Summary

INSTALLATION.md's Build from source section listed git, go and make
as the only dependencies, but the install target that the section tells you to
run pulls in the web UI first, and that needs Node.js and pnpm:

.PHONY: build-webui
build-webui:
	cd webui && pnpm install && pnpm run build

.PHONY: install
install: build-webui

So the documented steps fail on a minimal system at make install with pnpm
missing. This adds the two missing dependencies to the list, points at
.tool-versions for the pinned versions, and answers the second half of the
issue: building without the web UI is already possible, it just isn't written
down.

Changes

INSTALLATION.md and //:Makefile.

• nodejs and pnpm added to the dependency list, marked as web-UI-only, with
  a pointer to the new section below. Also links .tool-versions (which pins
  golang, nodejs, pnpm and goreleaser) and the asdf install shortcut,
  matching how CONTRIBUTING.md points at the same file.
• New nowebui target in //:Makefile that runs go generate +
  go build without the webui tag, so it needs no Node.js toolchain.
• New "Build without the web UI" section under Build from source, plus its
  entry in the mdformat TOC. It documents make nowebui as the alternative to
  make install.

nowebui builds, it does not install

nowebui is go build, not go install, so it is consistent with the other
targets in the file:

Only install writes to GOPATH/bin, which is what the surrounding
Build from source section already tells you to put on your PATH. The new
section therefore ends with a mv into that directory rather than claiming the
target did it for you.

Why building without the web UI works

The embedded assets sit behind a build tag, so the package compiles with an empty
asset set when the tag is absent:

// webui/assets.go
//go:build webui
//go:embed all:dist
var assets embed.FS

// webui/assets_stub.go
//go:build !webui
const Available = false

That behaviour is already documented in CONTRIBUTING.md; this makes
INSTALLATION.md agree with it instead of contradicting it, and gives it the
pointer from the issue.

Verification

make nowebui on a fresh clone with no node_modules and no pnpm install:

$ make nowebui
Generating documentation...
  - ManPage: ok
  - Markdown: ok
Generating completion files ...
  - ZSH: ok
  - PowerShell: ok
  - Bash: ok
  - Fish: ok
go build -ldflags "-X main.version="v0.11.0-44-g94e69a4-dirty"" .

$ ls git-bug
git-bug

$ git-bug version
git-bug v0.11.0-44-g94e69a4-dirty 94e69a419603/dirty go1.26.0 linux amd64

$ git-bug webui
Error: this binary was built without the web UI

The web UI is a separate frontend that has to be built with Node.js and
compiled into the binary. Binaries produced by "go build" or "go install" do
not include it.
...

Confirmed alongside the binary: nothing was written to $(go env GOPATH)/bin,
and mv git-bug "$(go env GOPATH)/bin" — the instruction the new section gives —
works.

One caveat is called out in the new section rather than left for users to trip
over: git bug version reports undefined for a binary built without the
Makefile, because getVersion() in //:version.go runs git describe output
through semver.IsValid after adding a v prefix, so only an exact release tag
produces a version string. Going through make nowebui keeps the -ldflags
stamping and reports v0.11.0-44-g94e69a4-dirty above, so this only affects
anyone who bypasses the Makefile.

Because go generate is part of the target, and the CI Check generated files
are up to date step regenerates doc/man, doc/md and misc/completion:
those three paths are byte-identical after running go generate without the
webui tag, so dropping the tag does not churn committed generated output.

CI

• gofmt -l . clean (no Go source changed).
• Generated-file freshness gate unaffected, as above.
• make build-webui, the pnpm test suite and make secure are untouched.

Refs #1578

Confidence Score: 5/5

The PR appears safe to merge; the remaining documentation error is non-blocking.

Findings

1.  Wrong command in release warning ▶

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