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/strings.go

@@ -0,0 +1,23 @@
+package cli
+
+// Shared CLI vocabulary used across multiple command definitions in this
+// package. Centralising the strings prevents goconst drift and ensures a
+// typo in a subcommand name fails to compile rather than silently
+// breaking the binding.
+const (
+	// Subcommand verbs (cobra Use field).
+	cmdList   = "list"
+	cmdShow   = "show"
+	cmdNew    = "new"
+	cmdDelete = "delete"
+	cmdExpire = "expire"
+
+	// Subcommand aliases.
+	aliasDel = "del"
+	aliasExp = "exp"
+
+	// Output table column headers and printOutput map keys.
+	colResult     = "Result"
+	colCreated    = "Created"
+	colExpiration = "Expiration"
+)