fix(client): preserve tab content across split panes

[openai0229]

Summary

• mount workspace tab bodies once in a stable content layer keyed by tab ID
• keep the recursive SplitPane tree responsible only for tab headers and measurable pane slots
• position active tab bodies over their pane slots and synchronize geometry with ResizeObserver during split, collapse, and resize operations
• preserve editor, result-view, and terminal component instances when tabs move between panes
• keep copy-split semantics independent: only the new copy mounts while the source tab remains untouched
• add focused tests for pane geometry and active tab-to-pane placement

Root cause

Workspace tab bodies were rendered inside each pane-specific Tabs component. Creating or collapsing a SplitPane changed their React parent hierarchy, which unmounted and remounted editors, query results, and terminals even when the tab IDs did not change. Batching the store update prevented an intermediate state but could not preserve component instances.

Validation

• yarn test:workspace-split-lifecycle
• yarn test:workspace-tab-drag
• yarn test:terminal
• yarn test:editor-close
• yarn test:community-boundary
• targeted ESLint and Stylelint
• git diff --check
• Community JCEF Webpack compilation and manual split/move/copy verification

Out of scope

The existing visual header flash when the last tab closes and a pane collapses is intentionally unchanged.