feat(webui): zoom fills the window, covering the session list

[edwin-zvs]

Web zoom only hid sibling panes, so the same gesture that clears the screen in the TUI left the session list occupying a column here — and with one pane it did nothing at all, which is why it was gated behind a split layout and echoed "nothing to zoom".

Zoom now means what it means in the TUI: the focused session view fills this client's window. The list gets out of the way along with the other panes, so zoom is worth offering with or without splits, and the split gate is gone.

Header and composer stay — zoom covers what competes with the view, not what drives it.

The two decisions worth reviewing

Zoom borrows the list; it does not consume the preference. The list has its own persisted show/hide setting. Zoom hides the list via its own class and never writes that setting, so unzooming restores whatever the user had rather than whatever zoom left behind.

Asking for the list is asking to leave zoom. Otherwise the ≡ toggle and C-x b are dead controls while zoomed — pressed, nothing happens, and the button's own label is lying about what a press does. The TUI answers the same request by zooming the list instead; there's no zoomed-list surface here, so leaving zoom is the honest answer. ≡ also reports the state zoom actually produced, not the stale preference.

Also dropped: the "collapsing to one pane clears zoom" guard from #1316. It existed because one pane meant nothing to zoom; that's no longer true.

Testing

crates/e2e/tests/split_layout.rs extended, in a real browser:

• zooming (split layout) takes the session list's width to 0, and unzooming gives it back;
• with a single pane the zoom row is enabled and still covers the list;
• clicking ≡ while zoomed brings the list back.

cargo test --workspace green.

Spec

New specs/0212-zoom-fills-the-client-window.md: what zoom means, that it is available without splits, that it must not consume the list preference, and that a control asking for the list must not do nothing. "Zoom" now has to mean one thing in both clients, and 0118 only covers where zoom lives (per-client), not what it does.

Relevant binary: construct — the daemon embeds index.html, so this ships in .claude/worktrees/webui-zoom-covers-list/target/debug/construct.

_{The screenshot commit is removed in the next commit; the raw link is pinned to the SHA that has it.}