CmdPal: Fix hotkey navigation when palette is showing transient dock page (#48089)

## Summary

When a keyboard shortcut opens CmdPal to an extension while the palette
is already showing a dock-launched transient page, `GoHome(false)`
cannot restore the root page — the frame's back stack is empty because
the transient dock page was never pushed on top of root. The user ends
up with only the hotkey-target page in the frame with no way to navigate
back to the main list.

## Root Cause

In `ShellPage.SummonOnUiThread()`, the hotkey-to-page branch called
`GoHome(false)` before sending `ShowWindowMessage`. But when the active
page is a transient dock page, `_currentlyTransient` is still `true` and
the frame back stack is empty, so `GoHome` can't re-establish the root
page as the frame base.

## Fix

Added `ResetToHome()` to `ShellViewModel`, mirroring the pattern already
used in `WindowHiddenMessage` handling:
1. Clears `_currentlyTransient`
2. Calls `_rootPageService.GoHome()` to reset extension state
3. Sends `PerformCommandMessage` for `_rootPage` — navigating
MainListPage into the frame as the base

In `ShellPage.SummonOnUiThread()`, the `GoHome(false)` call in the
`isPage` branch is replaced with `ViewModel.ResetToHome()`. The root
page is then cleanly in the frame before the hotkey target's
`PerformCommandMessage` navigates on top of it.

Fixes #47994

Co-authored-by: root <root@io.bbq>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>