Phase 0: remove auto-updater, pin Go 1.27.1, fix vet warnings, tidy ignores

- Delete BinaryUpdate, internal/up2date, GitHub/Update structs and the
  xteveAutoUpdate / update.url settings (UI rows, en.json, defaults).
  Settings-schema migrations kept and moved to src/migrate.go.
- Drop kardianos/osext dependency.
- xteve.go version 0200 -> 0201 to match changelog; Drone now fails on drift.
- go.mod go 1.27.1; Dockerfile and Drone golang images pinned to 1.27.1.
- Fix four go vet unreachable-code warnings.
- .gitignore: .gocache/, agent.md, skill.md. .dockerignore: build context
  no longer includes caches, ts/, tasks/ or markdown except the changelog.
- Drone: publish :latest only from master; other branches publish a
  branch-named tag so a feature push cannot replace the deployed image.
- Add tasks/improvement-plan.md and tasks/todo.md.
- Regenerate src/webUI.go.

html/js/base_ts.js

@@ -21,7 +21,7 @@ menuItems.push(new MainMenuItem("log", "{{.mainMenu.item.log}}", "log.png", "{{.
 menuItems.push(new MainMenuItem("logout", "{{.mainMenu.item.logout}}", "logout.png", "{{.mainMenu.headline.logout}}"));
 // Kategorien für die Einstellungen
 var settingsCategory = new Array();
-settingsCategory.push(new SettingsCategoryItem("{{.settings.category.general}}", "xteveAutoUpdate,tuner,epgSource,api,use_plexAPI,plex.url,plex.token"));
+settingsCategory.push(new SettingsCategoryItem("{{.settings.category.general}}", "tuner,epgSource,api,use_plexAPI,plex.url,plex.token"));
 settingsCategory.push(new SettingsCategoryItem("{{.settings.category.files}}", "update,files.update,temp.path,cache.images,xepg.missing.epg.mode,xepg.replace.missing.images"));
 settingsCategory.push(new SettingsCategoryItem("{{.settings.category.streaming}}", "buffer,udpxy,buffer.size.kb,buffer.timeout,user.agent,ffmpeg.path,ffmpeg.options,vlc.path,vlc.options"));
 settingsCategory.push(new SettingsCategoryItem("{{.settings.category.backup}}", "backup.path,backup.keep"));