fix(i18n): add missing translation keys and remove orphans (#1306)

[steilerDev]

Summary

• Adds 10 Tier-1 keys missing from both en and de locales, 9 Tier-2 keys missing from en only, and 1 Tier-3 de key across the budget, diary, householdItems, schedule, settings, and workItems namespaces
• Removes 3 orphaned keys no longer referenced in production code (2 from de, 1 from en)
• Hardens the translator agent definition with a Full Coverage Audit protocol to prevent recurrence

Fixes #1306

Note: After this PR, the audit tool will still show 2 missing_in_de and 7 missing_everywhere entries — these are pre-existing gaps introduced by the Budget Sources feature on beta HEAD and are out of scope for this fix. They should be tracked as a follow-up issue.

Test plan

• Quality Gates pass (typecheck, unit tests, build)
• No test regressions — no source logic changed, only translation JSON files
• i18n audit tool shows missing_in_en=0 and missing_in_de=0 for the keys in scope for this fix

🤖 Generated with Claude Code

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[steilerDev]

[product-architect] APPROVE (posted as comment — cannot self-approve own PR)

Architecturally sound. Scope is limited to i18n dictionary JSON and translator agent notes — no namespace restructuring, no key renames that cross feature boundaries, no API/schema touchpoints.

Reviewed:

• Namespace placement of new keys is consistent with existing conventions (budget.invoices.*, dashboard.cards.budgetSummary.*, schedule.milestones.detail.*, settings.*, diary.*, householdItems.*). Nothing leaks across features.
• Key naming follows the established camelCase dotted-path pattern (e.g., subsidiesOversubscribed, inspectionOutcomeRequired, contactAdded/Updated/Deleted).
• Orphan deletions (budget.vendors.tableHeaders.actions, budget.form.buttons.delete, householdItems.table.headers.room) are internal cleanup and don't affect contract shapes elsewhere.
• overview.costBreakdown.area.unassigned regression fix is a straight restore — good catch.
• EN/DE parity restored for the Area feature (Tier-2) — no architectural concern; Area is the structured dimension per ADR-028 and these keys belong in their respective feature namespaces.

No concerns. Pre-existing E2E failures on beta (Budget Sources) are unrelated to this PR.