[Frontend][Board] Keep mutation failures visible beside load errors

[Chris0Jeky]

Closes #2430

Summary

• keep a board-load alert and a later, distinct mutation failure visible at the same time
• keep Retry attached only to the board-load alert
• preserve loaded Legacy and Paper board content, including the Paper empty-board local error

Verification

• npm ci --no-audit --no-fund
• npm test -- --maxWorkers=2 src/tests/views/BoardView.spec.ts src/tests/views/paper/PaperBoardView.spec.ts
• npm run typecheck
• npx eslint src/views/BoardView.vue src/views/paper/PaperBoardView.vue src/tests/views/BoardView.spec.ts src/tests/views/paper/PaperBoardView.spec.ts
• git diff --check

Scope/docs check: no canonical docs change is required.

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[Chris0Jeky]

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[Chris0Jeky]

Review gate (Codex credits exhausted, SC-9): one fresh-context reviewer confirmed both alerts are distinct sections with Retry bound only to the load alert, that a stale fetch cannot write another board's message into state.error (boardCrudStore generation guard at :237, route change clears both errors), and that the specs assert simultaneity with exact text. Frontend Unit green at this head. Verdict SHIP. Three LOWs declined as non-blocking: value-equality dedupe can hide a mutation error whose text equals the load error (toast still fires); two role=alert regions can announce together; #2496 touches the same lifecycle lines — it will need a rebase after this lands. Merging in the next batch.