[pratikbuilds]

Panel resizes from PanelHandle by pointer without a React render while dragging, by arrow keys, and resets on double-click or Enter. The main area keeps a minimum width and a narrow container gets an overlay. PanelContent with keepMounted keeps a hidden pane's scroll position and typed text. useResizable holds the width logic.

Stacked on #3: review only this commit. Part of corbitsdev#94.

Notes for review

• Width is a CSS variable written by useResizable; zero React renders while dragging (e2e counts renders), one save on release.
• Overlay uses a container query (row narrower than 48rem), not a viewport breakpoint, so a sidebar beside the row counts; the consumer marks the row @container.
• Layout rule: Panel is the last child of a @container relative flex row (no gap) holding a min-w-0 flex-1 main area and the panel.
• PanelContent value is controlled only: the consumer's tabs own which pane shows.
• Extra props: minWidth (320), mainMinWidth (440), defaultWidth; label on PanelHandle/PanelClose. README Chat table gains a Panel row.

Verification

• This commit passes bun run typecheck and bun run lint on its own (the stack is checked commit by commit).
• At the top of the stack: bun run check, bun run build, bun run dep-guard, bun run contrast-test and bun run test:e2e (Playwright specs and the Storybook axe sweep in light and dark) pass.

Closes corbitsdev#106

[pratikbuilds]

Moved upstream: corbitsdev#116