Add prevent_unload/allow_unload actions

[myabc]

Implements two Browser actions to arm and disarm a beforeunload guard, so the server can warn the user before they leave a tab with unsaved changes.

<turbo-stream action="prevent_unload"></turbo-stream>
<turbo-stream action="allow_unload"></turbo-stream>

Approach

• Uses addEventListener("beforeunload", …) with a single module-level named handler, not the window.onbeforeunload property. This composes with any other beforeunload listeners and removes only its own handler — assigning the property would clobber (and be clobbered by) other scripts.
• The handler calls preventDefault() (the modern trigger) and sets returnValue for broader browser support. No browser supported by Turbo 7.2+ displays the returnValue string, so the generic confirmation dialog is shown.
• prevent_unload is idempotent (re-adding the same handler reference is a no-op).
• No custom message attribute: modern browsers ignore custom onbeforeunload strings and show a generic dialog.

Changes

• Added prevent_unload / allow_unload to src/actions/browser.ts and registered them.
• Added test/browser/before_unload.test.js — arm, disarm, and idempotency cases (asserts event.defaultPrevented).
• Documented both under Browser Actions in the README.

Closes #24

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds two TurboPower Browser actions that let the server enable/disable a beforeunload guard so users get a browser-native warning dialog when leaving a tab with unsaved changes. This integrates as a composable beforeunload event listener (via addEventListener / removeEventListener) rather than clobbering window.onbeforeunload.

Changes:

• Implement prevent_unload and allow_unload actions using a shared module-level beforeunload handler.
• Register both actions in Browser action registration.
• Add browser tests covering arming, disarming, and idempotent re-arming, plus README documentation entries.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/browser/before_unload.test.js	Adds tests validating the beforeunload guard can be armed/disarmed and remains removable after repeated arming.
src/actions/browser.ts	Implements and registers prevent_unload/allow_unload via a named beforeunload event handler.
README.md	Documents the two new Browser actions in the public action list.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[marcoroth]

What do you think about set_beforeunload and clear_beforeunload?

[myabc]

Hi @marcoroth

What do you think about set_beforeunload and clear_beforeunload?

I considered those names as well. I guess that would make the underlying implementation clearer (and align better with other action naming).