Document Go 1.26.6 baseline

[aleksclark]

Summary

• document the maintained Go 1.26.6 baseline in the web UI build-tooling guide
• preserve the existing go.mod directive and setup-go go-version-file configuration, which already resolve CI and release builds to Go 1.26.6

Verification

• GOMAXPROCS=2 GOFLAGS=-p=1 make ci
• go mod tidy -diff
• git diff --check

Scope notes

• This repository has one maintained Go module (go.mod), already declaring go 1.26.6.
• CI, release, and Crush review workflows already consume go.mod through actions/setup-go; no redundant hard-coded version was added.
• Historical planning text and agent guidance that intentionally use broader Go-version compatibility language were left unchanged.