Move to Gitea: git.ryuvia.com/niklas/terdut-tui

The module path, the CI pipeline and the self-updater all named GitHub. They now
name the Gitea instance everything else already runs on.

The workflows are rewritten rather than translated, for the reason recorded in
ci.yaml: Gitea's runner image is ubuntu:22.04, whose nodejs is Node 12, so no JS
action runs there -- actions/checkout@v4 dies with a SyntaxError before doing
anything. Every step is shell and checkout is a plain clone, which this public
repo needs no credential for. upload-artifact/download-artifact are JS actions
too, and there is no artifact store here, so the job that builds the binaries is
the job that publishes them.

internal/updater keeps its release and asset types unchanged: Gitea's release
payload carries the same tag_name, and its attachments the same name and
browser_download_url, so only the URL, the Accept header and one error string
move. The asset naming in release.yaml is load-bearing for that matching.

This does strand already-installed binaries, which still poll api.github.com.
The GitHub repository is left in place and untouched, so they report themselves
up to date rather than erroring; its last release is the bridge, and crossing it
is a one-time manual download.

internal/updater/updater.go

@@ -12,7 +12,14 @@ import (
 	"strings"
 )
 
-const releaseAPI = "https://api.github.com/repos/yeniklas/terdut-tui/releases/latest"
+// Gitea's release payload carries the same tag_name, and its attachments the same name
+// and browser_download_url, so the types below are unchanged from the GitHub original.
+//
+// A binary installed before the move still polls api.github.com and will never see a
+// release published here. That GitHub repository is still in place, so such a build
+// reports itself up to date rather than erroring -- its last GitHub release is the
+// bridge, and crossing it is a one-time manual download.
+const releaseAPI = "https://git.ryuvia.com/api/v1/repos/niklas/terdut-tui/releases/latest"
 
 type release struct {
 	TagName string  `json:"tag_name"`
@@ -125,7 +132,7 @@ func fetchLatest() (*release, error) {
 	if err != nil {
 		return nil, err
 	}
-	req.Header.Set("Accept", "application/vnd.github+json")
+	req.Header.Set("Accept", "application/json")
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
@@ -134,7 +141,7 @@ func fetchLatest() (*release, error) {
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("GitHub API returned %s", resp.Status)
+		return nil, fmt.Errorf("Gitea API returned %s", resp.Status)
 	}
 
 	var rel release