tailscale configure systray --enable-startup=systemd generates unit tied to default.target instead of graphical-session.target

### What is the issue?

`tailscale configure systray --enable-startup=systemd` generates a systemd user unit tied to `default.target` instead of `graphical-session.target`. Since `default.target` can be reached before the graphical session's D-Bus is fully up, the systray process starts too early, fails to connect to the session bus, and exits immediately.

Environment:
- OS: Ubuntu 26.04 (GNOME, Wayland)

Generated unit (`~/.config/systemd/user/tailscale-systray.service`):

​```ini
[Unit]
Description=Tailscale System Tray
Documentation=https://tailscale.com/kb/1597/linux-systray
Requires=dbus.service
After=dbus.service
PartOf=default.target

[Service]
Type=simple
ExecStart=/usr/bin/tailscale systray

[Install]
WantedBy=default.target
​```

Relevant logs (`journalctl --user -u tailscale-systray -b`):

​```
juil. 01 12:36:40 CRVS-L-001 systemd[5154]: Started tailscale-systray.service - Tailscale System Tray.
juil. 01 12:36:40 CRVS-L-001 tailscale[21562]: 2026/07/01 12:36:40 starting
juil. 01 12:36:43 CRVS-L-001 tailscale[21562]: 2026/07/01 12:36:43 new state: Stopped
juil. 01 12:36:49 CRVS-L-001 tailscale[21562]: 2026/07/01 12:36:49 exiting
juil. 01 12:36:49 CRVS-L-001 tailscale[21562]: 2026/07/01 12:36:49 ipnbus error: context canceled
juil. 01 12:36:49 CRVS-L-001 tailscale[21562]: 2026/07/01 12:36:49 watchIPNBus: ipnbus error: context canceled
juil. 01 12:36:49 CRVS-L-001 systemd[5154]: tailscale-systray.service: Main process exited, code=exited, status=1/FAILURE
​```

Proposed fix — tie the unit to `graphical-session.target` instead:

​```ini
[Unit]
Description=Tailscale System Tray
Documentation=https://tailscale.com/kb/1597/linux-systray
Requires=dbus.service
After=dbus.service graphical-session.target
PartOf=graphical-session.target

[Service]
Type=simple
ExecStart=/usr/bin/tailscale systray
Restart=on-failure
RestartSec=2

[Install]
WantedBy=graphical-session.target
​```

With this change, the systray reliably starts on login/reboot.

### Steps to reproduce

1. Run `sudo tailscale set --operator=$USER`
2. Run `tailscale configure systray --enable-startup=systemd`
3. Check the generated unit: `cat ~/.config/systemd/user/tailscale-systray.service` — it's tied to `default.target`
4. Reboot the machine
5. Expected: tray icon appears in the top panel
6. Actual: tray icon is absent. `systemctl --user status tailscale-systray` shows the service exited with status 1 shortly after boot (`ipnbus error: context canceled`)

### Are there any recent changes that introduced the issue?

Not a regression, this appears to be present since the `--enable-startup=systemd` feature was introduced in Tailscale 1.96. The generated unit has likely never correctly accounted for session/D-Bus startup ordering.

### OS

Linux

### OS version

Ubuntu 26.04 LTS

### Tailscale version

1.98.8 tailscale commit: 1241b225bc798707d02db3570992625d3a16594f long version: 1.98.8-t1241b225b-g0520dfda5 other commit: 0520dfda5d034816c38a15a8661160eb9a6d5ac4 go version: go1.26.3 (tailscale/go e877d97384)

### Other software

_No response_

### Bug report

BUG-b4bc7bb18e7509b343a66176da55ee9f56dfbff8549e00199d1c7832c7ac3e86-20260701105307Z-9766db338f781edb

Start at the `tailscale configure systray --enable-startup=systemd` entry point and trace where `~/.config/systemd/user/tailscale-systray.service` is generated. Compare the generated unit with the proposed `graphical-session.target` ordering and startup behavior, then verify the service starts reliably on login and that the tray icon appears.

Written by the indexing model from the issue text.