fix(site/src): show chat instead of the right panel on small viewports - #28387

The task list sidebar (TasksSidebar) had a fixed width up to 320px and
never collapsed automatically. On narrow viewports it consumed most of
the available width, leaving the chat/apps PanelGroup almost no room.
Combined with missing min-w-0 on flex ancestors, the layout overflowed
instead of shrinking, which let the right-hand apps panel dominate the
screen while the chat panel on the left was squeezed to a sliver.

- Auto-collapse TasksSidebar below the md breakpoint (matches the
  existing useIsBelowMdViewport hook used elsewhere); users can still
  expand it manually.
- Add min-w-0 to the TaskPage main column and TaskTopbar header so
  they shrink to the available width instead of forcing overflow.
- Allow the topbar's action buttons to wrap instead of overflowing on
  narrow screens.

Below the lg breakpoint, the AgentChatPage shows either chat or the
right panel (Summary/Git/Terminal/etc.) full-width at a time, driven
by the right panel's open/closed state. That state is a persisted user
preference (localStorage), so a panel left open on a wide window used
to keep hiding chat the moment the window narrowed, with only a small
back arrow to recover.

Add a useIsBelowLgViewport hook and use it to suppress the right panel
the moment the viewport crosses into the narrow range, so chat is
shown by default. The suppression is transient rather than persisted:
widening back past the breakpoint, or explicitly opening a tab while
already narrow, both restore the real preference instead of losing it,
matching how the left TasksSidebar reacts to viewport changes instead
of overwriting the user's choice.

Exercise the explicit panel toggle in the narrow-viewport story so a
clearSuppression regression fails the interaction test, and type the
matchMedia mock with satisfies so DOM signature drift is caught.

…pers

Derive one canonical showSidebarPanel value from the persisted
preference and the narrow suppression, narrow the setter to a plain
boolean so the raw-vs-effective updater bug cannot be reintroduced,
share the media-query subscription between breakpoint hooks, and
extract a reusable matchMedia test helper.

…essed

Gate visualExpanded and the RightPanel isExpanded prop on the panel
being shown, so a panel expanded on a wide viewport no longer covers
chat after the window narrows (or after an explicit close). The shared
matchMedia stub now delegates unconfigured queries to the real
implementation so story environments keep truthful responsive behavior,
and a story covers expand-then-narrow via a simulated breakpoint
crossing.