Hide the command menu button while the mobile side panel is open (#23471)

On mobile the side panel covers the page, but the page header stays
mounted underneath. Its command menu button (`⌘K`, the `⋮` icon) sits at
the same coordinates as the panel's own close button, so the two icons
render on top of each other.

Measured on a 390x844 viewport with the AI chat open:

- `Command Menu` button at `x=346, y=8, 32x32`
- `Close side panel` button at `x=358, y=14, 24x24`

`SidePanelToggleButton` already hid itself for the command menu and
search pages, but the AI chat pages (`AskAI`, `ViewPreviousAiChats`) are
not in `COMMAND_MENU_SIDE_PANEL_PAGES`, so the button stayed and
overlapped.

## Change

Hide the button on mobile whenever the side panel is open, rather than
enumerating pages — the header is not reachable behind a full-screen
panel either way.

Layout customization mode is the exception and keeps it:
`alignWithSidePanelTopBar` deliberately repositions the button into the
side panel top bar there, so that path is preserved.

Desktop is unaffected.

## Testing

Three cases added to `SidePanelToggleButton.test.tsx` (hidden on mobile
with the panel open, kept on mobile in layout customization mode, kept
on desktop with the AI chat open); the `useIsMobile` mock is now
switchable per test. All 10 tests pass.

Verified in the browser at 390x844: with the AI chat open only `Close
side panel` remains in the top bar, and the button reappears once the
panel is closed.


---
_Generated by [Claude
Code](https://claude.ai/code/session_018gcsCQbuTMsyFWv874p25Q)_

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.gitignore

@@ -63,3 +63,10 @@ screenshots/
 
 # local graphify knowledge-graph output (never commit)
 **/graphify-out/
+
+# Local scratch scripts
+/.verify*.mjs
+/.shot*.mjs
+/.diag*.mjs
+/.compare*.mjs
+/.narrow*.mjs