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

CONTRIBUTING.md

@@ -13,7 +13,7 @@ frontend, bridged by [Wails v3](https://wails.io/).
 
 | Tool | Version |
 |------|---------|
-| Go | 1.25+ |
+| Go | 1.26+ |
 | Node.js | 22+ |
 | pnpm | 10+ |
 | Wails CLI | v3 — vendored, no install needed (`go tool wails3`) |