cmd/tailscale/cli: fix time in tailscale routecheck (#19956)

When running `tailscale netcheck`, the reported timestamp used to be
in UTC and formatted according to RFC 3339 with a `T` to separate the
date from the time:

	sfllaw@h2co3:~$ tailscale netcheck | head -n3

	Report:
		* Time: 2026-06-01T21:12:32.252620138Z

This is machine-readable time leaking out to the user interface. Times
in normal commands are formatted for humans to read:

	sfllaw@h2co3:~$ date
	Mon 01 Jun 2026 02:39:14 PM PDT
	sfllaw@h2co3:~$ journalctl -t tailscaled | tail -n1
	Jun 01 14:35:21 h2co3 tailscaled[3328921]: wgengine: sending TSMP disco key advertisement to 100.90.144.102
	sfllaw@h2co3:~$ timedatectl show
	Timezone=America/Los_Angeles
	LocalRTC=no
	CanNTP=yes
	NTP=yes
	NTPSynchronized=yes
	TimeUSec=Mon 2026-06-01 14:38:32 PDT
	RTCTimeUSec=Mon 2026-06-01 14:38:32 PDT
	sfllaw@h2co3:~$ uptime --since
	2026-05-15 07:37:45

This PR makes the times printed by the CLI commands consistent:

- For `tailscale routecheck`, it now prints local time as
  `2026-05-15 07:37:45-07:00`.
- For `netlogfmt`, it has always printed local time with a space,
  but now includes the time zone.
- All machine-readable outputs continue to be standard RFC 3339 in
  UTC, i.e. `--format=json`.

As part of a general cleanup, this PR also adds standard common
time.Format layouts as tstime constants.

Fixes #19928

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tsweb/request_id.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"time"
 
+	"tailscale.com/tstime"
 	"tailscale.com/util/ctxkey"
 	"tailscale.com/util/rands"
 )
@@ -47,7 +48,7 @@ func GenerateRequestID() RequestID {
 	// Now we return "REQ-2" version, where the "2" doubles as the year 2YYY
 	// in a leading date.
 	now := time.Now().UTC()
-	return RequestID("REQ-" + now.Format("20060102150405") + rands.HexString(16))
+	return RequestID("REQ-" + now.Format(tstime.NumericDateTime) + rands.HexString(16))
 }
 
 // SetRequestID is an HTTP middleware that injects a RequestID in the