Add dark mode with theme toggle and OpenSpec change

- Add @custom-variant dark in globals.css for class-based dark mode
- Add ThemeToggle component with localStorage persistence and system preference fallback
- Inject blocking inline script in layout to prevent flash on load
- Apply dark: variants across all components (layout, site-nav, home-dashboard, expense-workspace, paycheck-workspace, recurring-expense-manager) and page headers
- Create openspec/changes/theming-dark-mode with proposal, design, and tasks artifacts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

AGENT_HANDOFF.md

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+# Monthy Tracker Agent Handoff
+
+Use this file as the starting point for any new agent. It summarizes the repo state, the important files, and the non-obvious behavior so you can continue without re-discovering basics.
+
+## What this app is
+
+- Single-user, local-first monthly expense tracker.
+- Stack: Next.js App Router, Prisma, SQLite, Ollama for offline AI.
+- Main views: dashboard, add expense, income/paychecks.
+- Goal: track expenses and paychecks, then generate private monthly summaries and recommendations.
+
+## Current state
+
+- Dashboard data is loaded from `/dashboard?month=YYYY-MM`.
+- Monthly insight generation lives in `src/lib/insights.ts`.
+- The app is currently working with the local dev DB at `file:./prisma/dev.db`.
+- Seed data exists for February 2026 and March 2026.
+- The frontend shows the current month by default, so if data looks missing, check the active month selector first.
+
+## Important files
+
+- `src/components/home-dashboard.tsx` - dashboard UI, fetches dashboard and Ollama status.
+- `src/lib/dashboard.ts` - aggregates totals, breakdowns, recent expenses, and comparisons.
+- `src/lib/insights.ts` - builds the model prompt, calls Ollama, stores fallback/model insights.
+- `src/app/dashboard/route.ts` - dashboard API route.
+- `src/app/backup/database/route.ts` - SQLite backup endpoint.
+- `src/lib/date.ts` - month/date helpers; month filtering uses local date strings.
+- `prisma/schema.prisma` - data model source of truth.
+- `prisma/seed.ts` - local seed script.
+- `README.md` - setup and runtime notes.
+- `openspec/changes/monthly-expense-tracker-v1/` - design/spec/task source for the product.
+
+## Key behavior to know
+
+- Dashboard aggregates all records, then filters them to the selected month in memory.
+- `getCurrentMonthKey()` uses local machine time.
+- `generateMonthlyInsight()` in `src/lib/insights.ts`:
+  - builds a structured prompt from dashboard totals, category breakdown, recent expenses, and chart points
+  - expects strict JSON with `summary` and `recommendations`
+  - falls back when activity is sparse or Ollama is unavailable
+  - stores the final result in `MonthlyInsight`
+- The fallback message is intentionally safe and short when data is too sparse.
+
+## Known gotchas
+
+- If dashboard values appear missing, confirm the selected month matches the seeded data.
+- If Docker backup fails, check `DATABASE_URL`; the backup route reads the SQLite path from the env var.
+- Prisma migration ordering matters because migration folders are applied lexicographically.
+- In Docker Compose on Linux, `extra_hosts: host-gateway` is needed for host Ollama access.
+
+## Current enhancement target
+
+- The next useful improvement is making the generated monthly summary more helpful and more structured.
+- Best place to change it: `src/lib/insights.ts`.
+- Likely changes:
+  - strengthen the prompt with clearer sections or required observations
+  - improve the fallback logic for sparse months
+  - optionally store more structured fields in `MonthlyInsight`
+  - update the dashboard card rendering in `src/components/home-dashboard.tsx` if the output shape changes
+
+## Useful commands
+
+```bash
+npm run dev
+npx prisma migrate deploy
+npx prisma db seed
+npm test
+```
+
+If you need to verify the dashboard API directly:
+
+```bash
+curl "http://localhost:3000/dashboard?month=2026-03"
+```
+
+## Recommended next step for another agent
+
+1. Read `src/lib/insights.ts`, `src/lib/dashboard.ts`, and `prisma/schema.prisma`.
+2. Decide whether to improve the insight prompt, the stored data shape, or the dashboard rendering.
+3. Make the change, then verify with the dashboard API and a generated insight.