Debounce the settings-file write when zooming the edit page (BL-16763)

[hatton]

Problem

While the user zooms the edit page with Ctrl+mousewheel, Bloom writes the whole user.config
settings file to disk on every zoom step. A fast wheel spin produces many of those synchronous
writes per second, all on the UI thread. This is not the root cause of the multi-minute freeze in
BL-16762, but each write holds the UI thread longer, which widens the window in which the
host-to-browser and browser-to-host calls collide.

Fix

EditingView.SetZoom no longer writes the settings file. It still applies the zoom to the browser
at once, and it still updates Settings.Default.PageZoom in memory at once, so nothing the user
sees changes. A two-second timer now does the disk write, and each zoom step restarts that timer,
so a whole wheel spin costs one write instead of one write per notch.

The pending write is also flushed at once when the user leaves the Edit tab, when the main window
becomes inactive, and when the view is disposed. So a zoom made in the last two seconds before any of
those events is still saved.

Ref: https://issues.bloomlibrary.org/youtrack/issue/BL-16763

Devin review

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This change is 

[greptile-apps]

Greptile Summary

The PR debounces persistence of Edit-page zoom changes while applying each zoom immediately in memory and in the browser.

• Adds a two-second WinForms timer that coalesces successive zoom changes into one settings-file write.
• Flushes a pending zoom setting when leaving Edit, deactivating the main window, or disposing the view.
• Documents a recurring isolated-build tooling issue in PAPERCUTS.md.

Important Files Changed

Filename	Overview
src/BloomExe/Edit/EditingView.cs	Replaces per-step zoom persistence with a debounced timer and lifecycle flush points; no follow-up-eligible issue was established.
PAPERCUTS.md	Adds a historical note about a recurring build-copy collision during preflight.

_{Reviews (2): Last reviewed commit: "Wait two seconds, not one, before writin..." | Re-trigger Greptile}

[hatton]

[Claude Opus 5 from Hatton's machine during preflight] Consulted Devin on 2026-08-26 21:20 UTC up to commit cb70423. It reported no bugs and no Investigate flags. It raised two Informational items, both of which it judged low risk itself: that a zoom made in the last second before an abrupt kill is lost (a deliberate trade-off, and the write is flushed on tab change, deactivate, and dispose), and that the flush on window-deactivate sits next to the BL-6299 warning (that warning is about the model save, which runs Javascript; this one only serializes the settings). The second item is now answered by a code comment. Nothing was mirrored to the PR, because Informational items are not posted.

[hatton]

[Claude Opus 5 from Hatton's machine during preflight] Consulted Devin on 2026-08-26 21:45 UTC up to commit 26a41fc (the delay raised to two seconds). No bugs and no Investigate flags. Three Informational items, all judged low risk: the debounce timer stays on the UI thread, a pending zoom is lost on a hard crash (the deliberate trade-off), and the flush on window-deactivate sits next to the BL-6299 warning, which is about the model save and its Javascript. Nothing mirrored to the PR, because Informational items are not posted.