serve: write-failure UX — toast + snap-back (STORE_DIRTY etc.)

[andreiRS]

Part of the serve-board spec (docs/specs/serve-board.md), architecture in docs/adr/0016-board-server-reuses-core-in-process.md.

Effort

S — cross-cutting error UX.

What to build

Consistent write-failure UX across all board mutations: when a write fails (e.g. STORE_DIRTY from a dirty Store, or any error envelope), revert the optimistic change and surface a toast carrying the error code. No persistent banner — per-action toast only (the board stays a live mirror).

Acceptance criteria

• A forced write failure (e.g. dirtied Store git tree) reverts the optimistic change and shows a toast with the error code.
• Move, create, and edit all share the same failure handling.
• The board never displays a state that did not persist.

Blocked by

#19, #21, #22

[andreiRS]

Done on feat/serve-board. Green: e2155d1 (unified toast + snap-back across move/create/edit). Fix-up: 893c8f1 (hide the idle toast popover so no empty box paints when there's no toast).

Gates: tsc --noEmit clean (root + web), web build clean.

What landed:

• AC2 (shared failure handling): all three writes (move/create/edit) now funnel their catch through one handleWriteFailure(set, {key, err, drop, revert}) helper — common work (clear the delay timer, drop the pending entry, derive code/message, raise the toast) lives in one place; the per-type revert (relocate / remove placeholder / restore fields) is passed as a closure and keeps each path's stillOptimistic guard. The three near-identical MoveError/CreateError/EditError classes collapse to one WriteError.
• AC1 + edit-path toast visibility (the serve: card drawer — rendered-markdown body + inline edit #22 carry-over): the toast became a manual Popover so it joins the browser top layer and shows ABOVE an open showModal() drawer (a plain fixed div painted behind the modal backdrop). It's non-modal, so the drawer stays interactive.

Browser-verified end-to-end on the single-origin server:

• Forced failure (dirtied Store) on move, create, and edit each reverts the optimistic change, writes no commit, and shows a STORE_DIRTY toast carrying the code — verified the snap-back (card returns to source lane / placeholder removed / fields restored) and that the board never persists the rejected state (AC3).
• On an edit failure the toast is now visible above the open drawer (confirmed :popover-open true, painted on top); previously it was hidden behind the modal.
• Idle board paints no toast element (empty-popover box fixed).
• Happy-path move still commits and reconciles (no regression from the consolidation).

Known minor limitation (acceptable; can polish later): while the modal drawer is open, the toast is painted on top and fully readable, but its ✕ dismiss isn't pointer-clickable because a modal <dialog> makes everything outside it inert — the toast clears on the next write or once the drawer is closed.

Deferred (noted for the integration review, not blocking): a SUPERSEDE toast for a discarded edit (mirroring move's discardedOwnWrite) was intentionally skipped — the three ACs don't require it.