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

.gitea/workflows/desktop-assets.yml

@@ -48,7 +48,7 @@ jobs:
       SHA: ${{ github.sha }}
       REF_NAME: ${{ github.ref_name }}
       DEBIAN_FRONTEND: noninteractive
-      GO_VERSION: '1.25.0'
+      GO_VERSION: '1.26.0'
       npm_config_store_dir: /cache/pnpm-store
     steps:
       # The same set ci.yml's check job installs: the app is cgo, and