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.

src/struct-webserver.go

@@ -45,7 +45,6 @@ type RequestStruct struct {
 		UserAgent                *string   `json:"user.agent,omitempty"`
 		XepgMissingEPGMode       *string   `json:"xepg.missing.epg.mode,omitempty"`
 		XepgReplaceMissingImages *bool     `json:"xepg.replace.missing.images,omitempty"`
-		XteveAutoUpdate          *bool     `json:"xteveAutoUpdate,omitempty"`
 		SchemeM3U                *string   `json:"scheme.m3u,omitempty"`
 		SchemeXML                *string   `json:"scheme.xml,omitempty"`
 	} `json:"settings"`