feat: add fee workspace and polish AI presentation

.trellis/tasks/07-09-ai-fee-frontend/design.md

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+# AI Presentation And Fee Workspace Design
+
+## Scope
+
+This task keeps the current AI product surface and adds one new frontend-only fee workspace. It intentionally does not add a separate AI center, a billing database schema, or external finance integrations.
+
+## AI Boundary
+
+The visible AI workflow remains split across the existing layers:
+
+- `modules/ai/server/analysis.ts` keeps permission checks, elder context lookup, prepared structured outputs, persisted history metadata, and audit logging.
+- `modules/ai/components/ElderAiAnalysisDialog.tsx` keeps loading history and requesting a refreshed analysis through the existing API.
+- `modules/dashboard/components/DashboardHome.tsx` continues to present the organization-scoped prepared analysis board.
+- `modules/ai/components/KnowledgeManagementClient.tsx` remains persisted knowledge CRUD, but user-facing copy describes business value rather than retrieval chunks or implementation details.
+
+No external model provider is called by the visible analysis path. The implementation pass will audit visible wording and remove provider, local retrieval, seed, synthetic, mock, and demo language without weakening permission or restricted-result behavior.
+
+## Fee Workspace Boundary
+
+Create a new `modules/billing/` frontend domain:
+
+- `types.ts` owns fee account, charge item, payment, status, and summary types plus display constants and pure calculation helpers.
+- `lib/presentation-data.ts` owns the initial organization fee ledger used by the workspace.
+- `components/BillingWorkspaceClient.tsx` owns all browser-session interactions.
+
+Create routes:
+
+- `app/(app)/app/billing/page.tsx`
+- `app/(app)/app/[organizationSlug]/billing/page.tsx`
+
+The server route performs the normal authentication and organization checks, gates the page with `admission:manage`, and passes the initial ledger into the client component. No API route or database table is added.
+
+## Navigation
+
+Add `费用管理` to the operations group after `床位房间`. Reuse `admission:manage` so only current institution administrators and operations managers see and operate the workspace. Add a Lucide wallet/receipt icon through the existing sidebar icon map.
+
+## Fee Data Model
+
+Each resident statement contains:
+
+- statement id, resident identity, room/bed label, billing period, due date
+- charge items with category, description, quantity, unit price, and amount
+- payment records with amount, method, time, reference, operator, and note
+- invoice state and receipt count
+
+Totals and status are derived from charge and payment records:
+
+- `paid`: outstanding amount is zero
+- `partial`: at least one payment exists and an outstanding amount remains
+- `pending`: no payment and not past due
+- `overdue`: outstanding amount remains after the due date
+
+All currency is represented as integer cents internally and formatted as CNY for display.
+
+## Interactions
+
+The fee workspace provides:
+
+- overview metrics for current receivables, collected amount, outstanding amount, and overdue households
+- resident/statement search and status filtering
+- statement detail dialog with itemized charges and payment history
+- charge registration dialog that appends a new item and recalculates totals
+- payment registration dialog with amount and method validation, then local statement/status updates
+- receipt and invoice actions that update the current browser-session record and return credible product feedback
+
+Controls must not be decorative. Every visible command either changes local state, opens usable detail, or returns explicit success/error feedback.
+
+## Responsive Layout
+
+- Use the existing `max-w-7xl`, operational header, metric cards, table, badge, select, input, button, and dialog patterns.
+- Keep tables horizontally scrollable with explicit minimum widths.
+- Stack toolbar controls and dialog form fields at narrow widths.
+- Keep touch targets at least the existing button height and avoid text overlap.
+
+## Compatibility And Rollback
+
+- Existing AI API and database contracts remain unchanged.
+- No migration or production data backfill is required.
+- Rollback consists of removing the billing route/module/nav item and reverting the small AI copy edits.
+- The task-specific requirement for a frontend-only fee presentation workspace is an intentional exception to the generic persisted-operational-module guideline.