internal/tray: add profile switching submenu to tray menu

[alex-poor]

Summary

Adds a "Switch account" submenu to the system tray right-click menu, allowing the user to switch between Tailscale profiles without opening the main window. The currently active profile is shown with a bullet character (`●`) prefix, and the indicator updates whether the switch is initiated from the tray submenu or from the main window's profile dropdown.

Profiles are fetched once at tray startup via `tsutil.GetProfileStatus` and the submenu is populated in `Tray.Start`. The submenu only appears when there is more than one profile available.

Dependency on tray library fix

This PR depends on DeedleFake/tray#2, which fixes three bugs in `deedles.dev/tray` that were preventing this from working:

1. `MenuItem.AddChild` was calling `emitPropertiesUpdated` on the parent instead of the child, causing a nil pointer dereference and dropping the child's property notifications.
2. `MenuItem.setChildren` was mutating `children-display` without emitting it, so the desktop environment never learned that an item had become a submenu.
3. `MenuItem.SetProps` had a lock-order inversion with `dbusmenu.GetLayout` that caused a hard deadlock whenever `SetProps` was called concurrently with the DE fetching the menu layout.

While that PR is in flight, the trayscale `go.mod` has a temporary `replace` directive pointing at my fork at the relevant commit:

```
replace deedles.dev/tray => github.com/alex-poor/tray v0.1.11-0.20260407004037-2451d98fc544
```

Once the upstream tray PR is merged, the `replace` directive should be dropped and the regular `deedles.dev/tray` requirement updated to a new pseudo-version.

Implementation notes

• The "Switch account" submenu must be the first item added to the menu — there is some interaction between the submenu's children and sibling items added to the root menu after it that I wasn't able to fully chase down. The trayscale code includes a comment explaining this constraint.
• `SetActiveProfile` updates the indicator labels using `SetProps`. It is called from a goroutine so that D-Bus emits don't run on the GTK main thread, and it serializes itself with `Tray.m` so concurrent indicator updates can't interleave and leave two items marked active.
• `Tray.Start` now takes an optional `*tsutil.ProfileStatus` so the submenu can be built in the same code path as all the other menu items, avoiding the need to mutate the menu after `Start` returns.

Test plan

• Tray menu shows "Switch account" submenu when more than one profile is configured
• Tray menu does not show the submenu when only one profile is configured
• Switching via the tray submenu actually switches profiles and the bullet indicator follows
• Switching via the main window profile dropdown also updates the tray bullet indicator
• No deadlocks or freezes during the disconnect/reconnect cycle that follows a profile switch
• Respects `TRAYSCALE_PRIVATE` mode (profile names shown as `profile@example.com`)

🤖 Generated with Claude Code

[DeedleFake]

Thank you for the pull request. Overall I like it, but I'd prefer that the profile list go into a submenu. Could you update it to do that, please?

[alex-poor]

Force-pushed an updated version of this PR that uses a submenu (per your earlier feedback) and includes the `SetActiveProfile` indicator-sync logic. Getting the submenu to render correctly required three upstream tray library fixes — see DeedleFake/tray#2 — and the trayscale `go.mod` currently has a temporary `replace` directive pointing at my fork while that lands.

I've updated the PR description with the full context.