fix: preserve pane title and close button when Agents, Activity, or Review is empty

[wolfiesch]

What this changes

• Empty pane headers wrapped: Updated AgentsPane, ActivityPane, and ReviewPane early empty checks to return a flex column structure containing <PaneHeading ... /> followed by the <FamilyEmpty ... /> view instead of returning the unadorned <FamilyEmpty ... /> wrapper directly.
• No-store fallback header wrapped: Wrapped PaneContent's sessionId === null early check in the same header structure to ensure that the panel title and desktop close buttons are rendered.
• FamilyEmpty sizing: Updated the child FamilyEmpty view to use min-h-0 flex-1 rather than forcing h-full internally to prevent column scroll overflows inside the h-full wrapper.
• Regression Test: Created a new test file apps/web/test/pane-empty-states.test.tsx containing isolated stores via createInspectorStore to verify that PaneHeading titles and the custom close buttons are rendered in all four empty state cases.

Why

• Fixes a bug where empty pane states (such as an empty Agents, Activity, or Review pane) would lack titles and desktop close buttons, leaving the pane undismissable.

How you verified it

• Added pane-empty-states.test.tsx and ran it successfully: all 4 test cases verified the presence of pane labels and close button markup.
• Verified full suite typecheck and checks pass via pnpm check.
• Verified all 995 unit/integration tests pass via pnpm --filter @t4-code/web test.

Checklist

• Verified build passes
• Verified tests pass
• Verified visual layout for empty states

[Copilot]

Pull request overview

Fixes empty-state pane rendering so pane headers (title + trailing close control) remain visible even when Agents/Activity/Review panes (or the PaneContent store) are empty, preventing undismissable panes.

Changes:

• Wrap empty-state early returns in AgentsPane, ActivityPane, and ReviewPane with a PaneHeading + FamilyEmpty column layout.
• Update PaneContent’s sessionId === null || store === null fallback to render the same header structure.
• Make FamilyEmpty accept an optional className so empty states can participate correctly in flex layouts; add a regression test covering the four empty cases.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
apps/web/test/pane-empty-states.test.tsx	Adds regression tests asserting headers + close control render in all targeted empty states.
apps/web/src/features/panes/ReviewPane.tsx	Ensures Review empty state still renders PaneHeading (incl. trailing close).
apps/web/src/features/panes/PaneContent.tsx	Ensures no-store/no-session fallback renders PaneHeading + empty content.
apps/web/src/features/panes/FamilyEmpty.tsx	Adds className support to allow flexible sizing (min-h-0 flex-1) when wrapped.
apps/web/src/features/panes/AgentsPane.tsx	Ensures Agents empty state still renders PaneHeading (incl. trailing close).
apps/web/src/features/panes/ActivityPane.tsx	Ensures Activity empty state still renders PaneHeading (incl. trailing close).

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[wolfiesch]

The outer container uses h-full min-h-0 to fill the panel, while FamilyEmpty is adjusted to min-h-0 flex-1 to sit within the flex structure without causing column scroll/height overflow.