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.

hscontrol/templates/ping.go

@@ -17,7 +17,7 @@ type PingResult struct {
 	// Status is "ok", "timeout", or "error".
 	Status string
 
-	// Latency is the round-trip time (only meaningful when Status is "ok").
+	// Latency is the round-trip time (only meaningful when [PingResult.Status] is "ok").
 	Latency time.Duration
 
 	// NodeID is the ID of the pinged node.
@@ -27,7 +27,7 @@ type PingResult struct {
 	Message string
 }
 
-// ConnectedNode is a node currently connected to the batcher,
+// ConnectedNode is a node currently connected to the [mapper.Batcher],
 // displayed as a quick-ping link on the debug ping page.
 type ConnectedNode struct {
 	ID       types.NodeID
@@ -36,7 +36,7 @@ type ConnectedNode struct {
 }
 
 // PingPage renders the /debug/ping page with a form, optional result,
-// and a list of connected nodes as quick-ping links.
+// and a list of connected nodes ([ConnectedNode]) as quick-ping links.
 func PingPage(query string, result *PingResult, nodes []ConnectedNode) *elem.Element {
 	children := []elem.Node{
 		headscaleLogo(),
@@ -94,7 +94,7 @@ func pingForm(query string) *elem.Element {
 		attrs.Style: styles.Props{
 			styles.Display:    "flex",
 			styles.Gap:        spaceS,
-			styles.AlignItems: "center",
+			styles.AlignItems: cssCenter,
 			styles.FlexWrap:   "wrap",
 			styles.MarginTop:  spaceM,
 		}.ToInline(),
@@ -107,7 +107,7 @@ func pingForm(query string) *elem.Element {
 			attrs.Autofocus:   "true",
 			attrs.Style: styles.Props{
 				styles.Padding:      "0.75rem " + spaceM,
-				styles.Border:       "1px solid var(--hs-border)",
+				styles.Border:       cssBorderHS,
 				styles.BorderRadius: "0.375rem",
 				styles.Width:        "280px",
 				styles.MaxWidth:     "100%",