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/android-apk.yml

@@ -68,7 +68,7 @@ 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
       # The Go half wants the NDK; the Gradle half wants a platform.
       ANDROID_HOME: /cache/android-sdk

.gitea/workflows/ci.yml

@@ -36,7 +36,7 @@ concurrency:
   cancel-in-progress: true
 
 env:
-  GO_VERSION: '1.25.0'
+  GO_VERSION: '1.26.0'
   # Shared by all three Playwright consumers (@playwright/cli, e2e/'s
   # @playwright/test, frontend/'s Vitest provider).  See the browsers
   # step in job 2 for why that is not the whole story.
@@ -53,7 +53,7 @@ jobs:
   check:
     runs-on: ubuntu-latest
     container:
-      # Not golang:1.25 — this job runs `make ui-test`, which is Vitest
+      # Not golang:1.26 — this job runs `make ui-test`, which is Vitest
       # *browser* mode and needs a Chromium and its system libraries
       # anyway, so the "fast job needs no browser" split does not hold.
       # Not the Playwright image either: e2e/ pins @playwright/test

.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

.gitea/workflows/index-artifact.yml

@@ -68,7 +68,7 @@ jobs:
       # claim with a test behind it now (cmd/indexbuild/deps_test.go),
       # because the v3 migration quietly broke it and this job was where
       # that surfaced.
-      image: golang:1.25
+      image: golang:1.26
       # This host path must exist on the runner and be listed verbatim in
       # act_runner's container.valid_volumes. It holds explore-staging/
       # (counts.bin + state.json) and yj.db — the checkpoint that makes