[Frontend][Board] Clear stale load errors after background recovery (re-target of #2496)

[Chris0Jeky]

Summary

Re-target of PR #2496 (issue-2434/clear-realtime-error) onto main. #2496 was stacked on issue-2430/board-mutation-alert; that base had already landed on main as #2495, but the branch still existed, so gh pr merge 2496 merged into the stale base branch (merge c60d0156a) and never reached main. This PR carries exactly #2496's delta (BoardView.vue +16/-1, BoardView.spec.ts +69) from the same branch head.

• let a successful current-board background refresh clear the matching stale board-load error
• clear the matching shared store error without hiding a newer mutation failure
• guard against route changes, stale refreshes, and post-unmount completions

Closes #2434

Review

Identical diff to #2496, which received one fresh-context review (Codex credits exhausted, SC-9) with verdict SHIP; the MEDIUM test-coverage gap is tracked in #2523. See the review-gate comment on #2496.

Verification

Hosted ci-required on this head is the proof; the prior head's checks were green except the Smart CI shadow gate (planner base-mismatch, recorded on #2327).

Lesson recorded

Stacked PRs whose base branch already merged must be re-targeted (gh pr edit N --base main) before gh pr merge, or the merge lands on the dead base branch.

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