Hide window-link graphs while Overview runs

[epeicher]

Follow-up to #354 (which hid the graphs in split view) and #337 (window links).

What it does

The window-link splines now hide while Overview (Exposé) is active — fading out as the thumbnails animate in, and fading back in once the exit animation has settled. Previously the arrows stayed on screen anchored to the windows' pre-overview positions, floating in space and pointing at nothing.

Rationale

Overview lays windows out as scaled transform: translate(...) scale(...) thumbnails, but the link frame geometry reads offsetLeft / offsetWidth — metrics CSS transforms don't touch. During the mode every tie kept pointing at where the windows used to be. Re-anchoring the arrows onto the live thumbnails would require per-frame transform reads through the 280 ms enter/exit animations; hiding matches the split-view precedent (#354): modes where windows aren't at their real desk positions draw no ties.

Implementation

One new flag in the render host, folded into the existing visibility policy:

• overviewActive flips on at OVERVIEW_ENTERING — not ENTERED — so the layer's 0.25 s opacity fade runs alongside the thumbnail animation.
• It flips off at OVERVIEW_EXITED, which fires only after the exit animation settles (280 ms), so the re-shown ties anchor on real geometry, never mid-flight windows. Exit also emits a fresh frame: selecting a thumbnail maximizes it while the layer is hidden.
• applyVisibility() gains a ! overviewActive condition, so mid-overview recomputes (settings changes, focus churn, membership changes) can't resurface the layer while the mode runs.

Documented next to the windowLinkVisibility setting in docs/javascript-reference.md: the layers hide during Overview whatever the visibility setting.

Testing instructions

npm run test:js -- window-links-render-host

The new regression test fails on trunk and passes here.

Manual (two related windows — e.g. a post editor and one of its comments):

1. With both windows floating, confirm the spline draws between them.
2. Enter Overview (the dock/admin-bar Overview button). No arrows should be visible over the thumbnail grid.
3. Exit Overview (Escape, or pick a window). The spline reappears, anchored on the windows' real positions — including when the picked window maximizes.