build: require Go 1.26 everywhere at once

The newest go-json-experiment/json, which wails/v3's application
package imports, declares go 1.26, so taking it raises our go
directive with it.  Every other place that names a Go version moves in
the same commit: GO_VERSION in ci, desktop-assets and android-apk, the
Arch makedepends, and CONTRIBUTING's table.

index-artifact's container image moves too, and it is the one that
matters most.  The official golang images set GOTOOLCHAIN=local, so a
golang:1.25 container refuses a go 1.26 module outright rather than
fetching a newer toolchain, and that job owns the ~205 GB checkpoint.

Closes #265

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

packaging/arch/PKGBUILD

@@ -18,7 +18,7 @@ arch=('x86_64')
 url="https://git.ljones.me/yonlu/yellowjacket"
 license=('custom')
 depends=('webkitgtk-6.0' 'gtk4' 'alsa-lib' 'hicolor-icon-theme')
-makedepends=('go>=1.25' 'nodejs>=22' 'pnpm' 'git')
+makedepends=('go>=1.26' 'nodejs>=22' 'pnpm' 'git')
 options=('!lto')
 
 # Source is overridable so the same PKGBUILD works two ways: