cmd, templates, integration: extract shared production constants

Constants the operator/test reader benefits from centralising.
Tests stay verbatim (the .golangci.yaml goconst tune skips them);
extraction here applies only where the same literal acts as
shared vocabulary across files.

  cmd/headscale/cli/strings.go (new)
    Cobra subcommand verbs and aliases shared by every list / show
    / new / delete / expire command across api_key, nodes, policy,
    preauthkeys, users — plus the Result / Created / Expiration
    column headers used in printOutput maps.

  hscontrol/templates/design.go
    cssBorderHS, cssBreakWord, cssCenter, cssOverflowWrap — shared
    styles applied across design.go, ping.go, register_confirm.go.
    spaceS already existed; switch raw "0.5rem" literals to it.

  integration/hsic/hsic.go
    binHeadscale, flagOutput, acceptJSON — names invoked across
    hsic.go and config.go.

  integration/tsic/tsic.go
    tailscaleBin — used across docker exec call sites.

cmd/headscale/cli/root.go

@@ -30,7 +30,7 @@ func init() {
 		Bool("force", false, "Disable prompts and forces the execution")
 
 	// Re-enable usage output only for flag-parsing errors; runtime errors
-	// from RunE should never dump usage text.
+	// from [cobra.Command.RunE] should never dump usage text.
 	rootCmd.SetFlagErrorFunc(func(cmd *cobra.Command, err error) error {
 		cmd.SilenceUsage = false