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+# CLAUDE.md — Databrae
+
+## Project Overview
+
+Databrae is a PHP web application that generates printable library spine labels with Data Matrix 2D barcodes. It serves as a low-cost alternative to RFID for library inventory, shelf-reading, and weeding workflows. Licensed under GNU GPL v3.
+
+## Tech Stack
+
+- **Language**: PHP >= 8.1 (no framework — standalone)
+- **Frontend**: HTML5, CSS3, vanilla JavaScript
+- **Dependencies**: Composer — single dependency: `tecnickcom/tc-lib-barcode` ^2.4
+- **Required PHP extensions**: BCMath, GD, PCRE
+- **Database**: None (stateless, no persistence)
+- **Tests**: None currently
+
+## Project Structure
+
+```
+databrae/
+├── index.php          # Single label generator (simple form)
+├── index-csv.php      # Bulk CSV label generator with sheet templates
+├── composer.json       # Dependency config
+├── composer.lock
+├── vendor/             # Composer dependencies (do not edit)
+├── README.md
+└── LICENSE
+```
+
+## Key Files
+
+- **`index.php`** — Single-label mode. Form with 4 data fields + barcode field. Renders one Data Matrix barcode as inline SVG.
+- **`index-csv.php`** — Advanced mode. CSV upload (auto-detects delimiter), sheet template system with customizable page/label dimensions, pagination. Required CSV headers: `data1, data2, data3, data4, barcode`.
+
+## Coding Conventions
+
+- `declare(strict_types=1)` used in index-csv.php
+- **Variables**: snake_case (`$page_width_in`, `$csv_file`)
+- **Functions**: snake_case (`safe()`, `strip_bom()`, `detect_delimiter()`)
+- **CSS classes**: kebab-case (`.label-card`, `.barcode-block`)
+- All user input escaped via `safe()` helper wrapping `htmlspecialchars(ENT_QUOTES, 'UTF-8')`
+- Null coalescing (`??`) for safe defaults
+- Barcodes rendered as inline SVG (not images)
+- CSS uses physical units (`in`, `cm`) for print accuracy
+- `@media print` rules hide UI and preserve label dimensions
+
+## Common Tasks
+
+```bash
+# Install dependencies
+composer install
+
+# Run locally (PHP built-in server)
+php -S localhost:8000
+```
+
+## Architecture Notes
+
+- No routing — each PHP file is its own entry point
+- No database or session state — all processing is per-request
+- CSV files are processed in memory (10MB max upload)
+- Sheet template parameters passed via POST, with sensible defaults (Demco 1.5"×1.0" on Letter)
+- Error handling: validation errors collected in array, displayed as list; barcode generation wrapped in try-catch