feat: add Supabase vendor data integration (BSR, ratings, buy box)

- Create vendor_daily_data table schema and Supabase client service
- Add upload API route for Vendor Central CSV parsing and upsert
- Add VendorDataView with BSR trend, ratings, and buy box charts
- Integrate new Vendor tab into app navigation (desktop + mobile)
- Add vendor CSV upload card to FileUpload modal

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

docs/plans/2026-02-20-supabase-vendor-data-design.md

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+# Supabase Vendor Daily Data Integration
+
+## Overview
+
+Store daily Vendor Central export data (BSR, ratings, buy box) in Supabase. Upload via CSV, display aggregated to weekly in a new "Vendor Data" view.
+
+## Database Schema
+
+Single flat table with composite unique key `(date, market, asin)`:
+
+```sql
+CREATE TABLE vendor_daily_data (
+  id BIGSERIAL PRIMARY KEY,
+  date DATE NOT NULL,
+  market TEXT NOT NULL,
+  asin TEXT NOT NULL,
+  product_title TEXT,
+  tags TEXT,
+  bsr_top_rank INTEGER,
+  bsr_top_category TEXT,
+  bsr_detail_rank INTEGER,
+  bsr_detail_category TEXT,
+  avg_rating NUMERIC(3,1),
+  num_reviews INTEGER,
+  buybox_owner TEXT,
+  buybox_price NUMERIC(10,2),
+  amazon_has_buybox BOOLEAN,
+  glance_views INTEGER,
+  created_at TIMESTAMPTZ DEFAULT NOW(),
+  UNIQUE(date, market, asin)
+);
+
+CREATE INDEX idx_vendor_daily_market_date ON vendor_daily_data(market, date);
+CREATE INDEX idx_vendor_daily_asin ON vendor_daily_data(asin);
+CREATE INDEX idx_vendor_daily_tags ON vendor_daily_data(tags);
+```
+
+## CSV Column Mapping
+
+| CSV Column | DB Column | Transform |
+|------------|-----------|-----------|
+| Date | date | Parse as DATE |
+| Market | market | Direct (DE, UK, IT, FR, ES) |
+| ASIN | asin | Direct |
+| Product Title | product_title | Direct |
+| Tags | tags | Direct (product line) |
+| Top Level Category (Rank) | bsr_top_rank | Parse INT |
+| Top Level Category (Name) | bsr_top_category | Direct |
+| Detail Level Category (Rank) | bsr_detail_rank | Parse INT |
+| Detail Level Category (Name) | bsr_detail_category | Direct |
+| Average Rating | avg_rating | EU decimal (3,8 -> 3.8) |
+| Number of Reviews | num_reviews | Parse INT |
+| Buybox Seller Name | buybox_owner | Direct |
+| Buybox Price | buybox_price | EU decimal (2,49 -> 2.49) |
+| Amazon Has Buybox | amazon_has_buybox | 1/0 -> boolean |
+| Glance Views | glance_views | Parse INT |
+
+## New Files
+
+| File | Purpose |
+|------|---------|
+| `services/supabase.ts` | Client init + query helpers |
+| `api/upload-vendor-data.ts` | Parse CSV, upsert to Supabase |
+| `components/VendorDataView.tsx` | BSR/ratings/buybox charts |
+
+## Data Flow
+
+### Upload
+1. User clicks "Upload Vendor CSV" in FileUpload.tsx
+2. Client sends file to `/api/upload-vendor-data` (POST)
+3. API parses CSV, extracts 15 columns, converts EU numbers
+4. Upserts to Supabase in batches of 500
+5. Returns `{ inserted, updated }` counts
+
+### Display
+1. VendorDataView queries Supabase directly (anon key, read-only)
+2. Client aggregates daily -> weekly (AVG for ranks/ratings, latest for buybox)
+3. Renders Recharts charts: BSR trend, ratings, buy box status
+
+## View: VendorDataView.tsx
+
+Three chart panels:
+1. **BSR Trend** - Line chart, Y-axis inverted, lines per marketplace
+2. **Ratings & Reviews** - Dual axis: avg rating + review count
+3. **Buy Box Status** - % days Amazon vs Seller vs Other
+
+Filters: marketplace, product line, ASIN, date range (reuse MultiSelectDropdown).
+
+## Integration Points
+
+- **App.tsx**: New `vendor` view state + nav button
+- **FileUpload.tsx**: 4th upload card for Vendor CSV
+- **package.json**: Add `@supabase/supabase-js`
+- **Vercel env vars**: `SUPABASE_URL`, `SUPABASE_ANON_KEY`, `SUPABASE_SERVICE_KEY`
+
+## Environment Variables
+
+| Variable | Where Used | Purpose |
+|----------|-----------|---------|
+| SUPABASE_URL | Client + API | Supabase project URL |
+| SUPABASE_ANON_KEY | Client | Read-only access |
+| SUPABASE_SERVICE_KEY | API routes only | Write access for upserts |
+
+## Security
+
+- Anon key for client reads, service key for server writes
+- No RLS (private internal tool)
+- Env vars in Vercel, not hardcoded