Board store needs operation-owned loading state instead of one shared Boolean

[Chris0Jeky]

Summary

Every board sub-store currently writes the same state.loading Boolean directly. Independent asynchronous operations can overlap, so whichever operation settles first clears loading for all of them—even when another request still owns a visible board/list load.

Confirmed navigation race

1. A mutation for board A begins and sets state.loading = true.
2. The user navigates to board B; its detail read begins and also sets the same Boolean to true.
3. The board-A mutation resolves first. Its finally writes state.loading = false.
4. Board B is still on the wire, but BoardView/PaperBoardView lose skeleton or retry-disabled state early and may expose stale/empty detail until the read settles.

The same ownership failure exists between overlapping board-list/detail reads and card, column, label, or comment mutations. Post-await board-state guards do not fix the flag because each finally still clears it unconditionally.

Expected design

Replace direct shared-Boolean ownership with one store-level operation mechanism, such as epoch-aware loading tokens or a reset-safe active-operation set:

• starting an operation returns its own token;
• settling removes only that token;
• loading remains true while any current-epoch token exists;
• logout/reset invalidates old tokens and clears the visible state synchronously;
• stale/cancelled generations cannot strand loading true or clear a newer session's owner;
• board-list/detail generation rules and existing mutation return/error behavior remain intact.

Do not solve this with scattered board-id conditions: list reads, detail reads, and mutations overlap across different contexts.

Acceptance

• Deferred-promise regressions for old-board mutation vs new-board detail load, overlapping mutations, and logout/reset with an old request still settling.
• Existing list-share, detail-generation, queued-background-refresh, and cancellation tests remain green.
• All direct state.loading.value = true/false writes in board action modules either move behind the shared owner or are explicitly justified as reset/demo boundaries.
• Frontend lint, typecheck, build, full tests, and exact-head hosted CI pass.

Audit provenance

Found during the sibling async-integrity pass that produced #3299, #3301, and #3303. Kept separate because it is a cross-module ownership invariant rather than a local fix.

[Chris0Jeky]

Backlog/admission note from the 2026-09-20 audit: this is deliberately not folded into #3300/#3302/#3304. Those PRs repair local ownership defects; replacing the shared Boolean is a cross-module state contract spanning board list/detail reads and every mutation store. Sequence it with #3306 so reset invalidation and operation-token settlement share one epoch rather than creating two competing mechanisms. No active branch currently owns these paths.