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/tui/update.go

@@ -7,7 +7,7 @@ import (
 	"github.com/atotto/clipboard"
 	"github.com/charmbracelet/bubbles/viewport"
 	tea "github.com/charmbracelet/bubbletea"
-	"github.com/yeniklas/terdut-tui/internal/api"
+	"git.ryuvia.com/niklas/terdut-tui/internal/api"
 )
 
 func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {