Toast a status note only for a block the front-end holds eager

[nbenn]

Summary

• Since Report a parked block's last check and return its result #364 the check that finds a block unable to run records its status-phase note, whatever asked for the check: an evaluate request, a downstream read of the block as input, or "Evaluate blocks" in the code modal. The notify_user() plugin toasted every such warning with no duration, so a block nobody was looking at raised a toast that stayed until the block was fixed.
• The plugin now toasts a status-phase note only for a block the front-end holds eager, the test Render a block when the front-end claims it, not when visible reports it painted #366 moves rendering onto, or for any block on a board no front-end gates. The note is still recorded by every check, for the status badge and the assistant, and conditions from other phases, such as a failed block's error, toast as before.
• The plugin receives board but not which owner is the front-end, so the board now carries the front-end's eager set as a front_end_eager reactive, TRUE on an ungated board. A block's notification observer reads it only while the block carries a status note, so a change to the eager sets re-runs only those observers.
• On core's own stacked board, "Evaluate blocks" with unconfigured blocks in a collapsed stack now toasts only those in the open stack. Expanding the stack brings the others' toasts up, and collapsing it removes them again.

Fixes #370

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
R/block-server.R	96.68% <ø> (ø)
R/board-server.R	97.68% <100.00%> (+<0.01%)	⬆️
R/plugin-notification.R	100.00% <100.00%> (ø)

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[nbenn]

Closing without merging. Which blocks are on screen is for the front-end to know, so a filter on toasts belongs in the front-end's own notification plugin rather than in a board field that reads vis$gate. Core's own plugin stays as it is.