refactor: use static toast label keys

[Chris0Jeky]

Refs #1991

Summary

• Replace PaperToastContainer's runtime-assembled toast translation key with an exhaustive static Record<ToastLabel, string>.
• Preserve all existing label text, severity tones, and locale behavior.

Verification

• PaperToastContainer.spec.ts: 23/23
• Frontend typecheck: passed
• File-scoped ESLint: passed
• Exhaustiveness mutation: removing the ailed entry produced TypeScript TS2741; the map was restored and the focused suite returned to 23/23.
• git diff --check: passed

Two coordinator review passes completed for contract correctness and scope/regression quality.

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[Chris0Jeky]

CI note (coordinator, 2026-09-05): the only red at head 28890ff was Backend Unit (windows-latest) in run 33941869440: Taskdeck.Application.Tests.Services.ArtefactExtractionServiceTests.ExtractAsync_ShouldPropagateCallerCancellationWithoutRecording failed after 5 s. This PR changes one Vue component and no backend file, so the failure cannot come from the diff; it is a backend cancellation-propagation timing case on the Windows runner with no tracked issue yet. One diagnostic rerun of the failed job started; the flake gets its own issue once the rerun result is in.

[Chris0Jeky]

Review gate (Codex review credits exhausted, SC-9; D-12 ruled 2026-09-05: the Codex session's PRs are reviewed and merged by the coordinator): one fresh-context read-only reviewer (Opus 5) on head 28890ff63. Verdict: SHIP.

Checked by reading: the ToastLabel union (store/toastStore.ts:30-38) has exactly eight members and TOAST_LABEL_KEYS maps all eight and no extras, so Record<ToastLabel, string> is exhaustive (a missing member is TS2741); all eight toast.label.* children exist in locales/{en,es,it}/shell.ts; the removed template-key line produced byte-identical keys for every reachable input, so no behavior change; the existing spec resolves real messages for all four explicit labels, all four severity fallbacks and one non-English locale, so a wrong key string would fail the suite; one file, +13/-1, no drive-by edits.

Findings and disposition:

• LOW, PaperToastContainer.vue:90-100: the doc comment calls each entry a "checked catalog key", but the value type is plain string, so TypeScript only checks member presence, not catalog existence. Tracked on #1991; no code change owed for merge.

Issue disposition: slice of #1991 (the runtime-assembled i18n key item only); the board-scoped eyebrow divergence, unreserved applied label and refusal-toast stamping residuals stay open.