feat: add Filament admin panel with migrations and design spec

- Add AdminPanelProvider mounting panel at `/admin` with `is_admin` auth guard
- Add `is_admin` boolean column to users table
- Add brent_prices and price_predictions tables with appropriate indexes
- Add comprehensive admin design spec covering resources, dashboard, navigation, and build order
- Configure default panel with amber primary color and standard middleware stack
- Add compiled Filament assets (actions.js, app.css)

public/js/filament/forms/components/textarea.js

@@ -0,0 +1 @@
+function n({initialHeight:e,shouldAutosize:i,state:h}){return{state:h,wrapperEl:null,init(){this.wrapperEl=this.$el.parentNode,this.setInitialHeight(),i?this.$watch("state",()=>{this.resize()}):this.setUpResizeObserver()},setInitialHeight(){this.$el.scrollHeight<=0||(this.wrapperEl.style.height=e+"rem")},resize(){if(this.$el.scrollHeight<=0)return;let t=this.$el.style.height;this.$el.style.height="0px";let r=this.$el.scrollHeight;this.$el.style.height=t;let l=parseFloat(e)*parseFloat(getComputedStyle(document.documentElement).fontSize),s=Math.max(r,l)+"px";this.wrapperEl.style.height!==s&&(this.wrapperEl.style.height=s)},setUpResizeObserver(){new ResizeObserver(()=>{this.wrapperEl.style.height=this.$el.style.height}).observe(this.$el)}}}export{n as default};