feat: add embedded database support for static deployments

Implements #10

Allows users to embed .db files at build time by placing them in public/databases/ directory.
Perfect for deploying DuckUI with demo data or creating self-contained analytical dashboards.

Features:
- Auto-loads databases defined in public/databases/manifest.json
- Fetches and registers .db files in DuckDB's virtual file system
- Attaches databases automatically on startup
- Graceful error handling with console logging
- Full documentation in public/databases/README.md

Usage:
1. Place .db files in public/databases/
2. Add entry to manifest.json with name, file, description, autoLoad
3. Build and deploy - databases load automatically

Example manifest.json:
{
  "databases": [
    {
      "name": "Demo DB",
      "file": "demo.db",
      "description": "Sample data",
      "autoLoad": true
    }
  ]
}

Author: caioricciuti

public/databases/README.md

@@ -0,0 +1,70 @@
+# Embedded Databases
+
+This directory allows you to embed DuckDB database files (`.db`) that will be automatically loaded when DuckUI starts. This is perfect for:
+
+- Deploying DuckUI with demo data
+- Distributing pre-configured databases
+- Creating self-contained analytical dashboards
+
+## How to Add Your Database
+
+1. **Place your `.db` file in this directory**
+   ```
+   public/databases/my-database.db
+   ```
+
+2. **Register it in `manifest.json`**
+   ```json
+   {
+     "databases": [
+       {
+         "name": "My Database",
+         "file": "my-database.db",
+         "description": "Description of your database",
+         "autoLoad": true
+       }
+     ]
+   }
+   ```
+
+3. **Build and deploy**
+   ```bash
+   bun run build
+   ```
+
+## Manifest Format
+
+Each database entry in `manifest.json` supports:
+
+- **`name`** (required): Display name in the UI
+- **`file`** (required): Filename of the `.db` file in this directory
+- **`description`** (optional): Description shown in the UI
+- **`autoLoad`** (optional, default: `true`): Whether to load on startup
+
+## Example
+
+```json
+{
+  "databases": [
+    {
+      "name": "Sales Demo",
+      "file": "sales-demo.db",
+      "description": "Sample sales data from 2023",
+      "autoLoad": true
+    },
+    {
+      "name": "Analytics",
+      "file": "analytics.db",
+      "description": "Web analytics data",
+      "autoLoad": false
+    }
+  ]
+}
+```
+
+## Notes
+
+- Database files are fetched and loaded in the browser
+- Large database files will increase initial load time
+- All databases are loaded into memory (consider file sizes)
+- You can attach/detach databases dynamically from the SQL editor

public/databases/manifest.json

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+{
+  "databases": []
+}