Reflow terminal panes when a sibling is spawned

[luckeyfaraday]

Problem

Spawning a terminal pane below an existing one didn't shrink the pane above — the new pane rendered into a stale, half-garbled layout. It only happened after a pane had been manually resized.

Root cause

Each pane exposes a native drag-resize handle (.slotPane { resize: vertical }). When you drag it, the browser writes an inline height: straight onto the DOM node. React never sets that property, so it never clears it on re-render — the resized pane stays pinned at its old height and won't participate in the grid when a sibling is added. Un-resized panes have no inline height, so they reflow fine (hence "only sometimes").

Fix

• CommandRoom.tsx — clear the inline height on every visible pane whenever the pane set changes (spawn / close / reorder), so the grid re-tiles all of them. Because each pane's box then actually changes size, its existing ResizeObserver fires and refits the xterm, clearing the stale render too.
• styles.css — switch the non-focused embedded stage to an equal share-space grid (grid-auto-rows: minmax(0, 1fr) + align-content: stretch, .slotPane min-height: 0) so panes shrink to make room instead of the stage just scrolling.

Behavior / trade-off

Manual vertical resize still works within a stable set of panes; it's just no longer sticky across spawn/close — adding or removing a pane snaps everyone back to equal tiling. overflow: auto is kept so a deliberately tall pane scrolls rather than clips.

Testing

• tsc -p tsconfig.json --noEmit passes.
• Not yet driven in the running Electron UI.

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