Make dialog collapsed and maximized states mutually exclusive

[ThresholdOps]

Description

Enforces the dialog state invariant discussed in #1418: a dialog should not be both collapsed and maximized at the same time.

The reducer now clears maximized when a dialog is collapsed, and clears collapsed when a dialog is maximized. This prevents the remaining blank fullscreen dialog state caused by a maximized wrapper around a collapsed dialog (open the stylesheet/JavaScript pane, maximize it, then collapse it).

Adds reducer coverage for both state transitions.

Notes for reviewer

Two things noted but intentionally not changed here — this PR sells one thing: the !(collapsed && maximized) invariant.

• passage-edit-stack.tsx's collapsed && maximized style branch becomes dead code once that state can't occur. Left as-is; normal collapsed styling is unaffected.
• setDialogMaximized already clears maximized on all dialogs for an out-of-range index (the existing incorrect-index test doesn't exercise this). Pre-existing; untouched.

Issues Fixed

Addresses the remaining dialog state issue discussed in #1418.

Using "Addresses" rather than "Fixes" pending a manual UI pass on the stylesheet/JavaScript panel, a single passage editor, and a passage stack with several passages.

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