Restyle the web UI in a light theme, step 1: theme, top nav, demo trading

Follow the Stitch "Daylight" design: light canvas, white panels with
hairline borders, deep cyan primary, Inter for UI text and JetBrains Mono
for numbers (bundled via @fontsource-variable).

- Override Element Plus variables in one place and derive light/dark
  shades with color-mix; pages use the UnoCSS Tailwind palette.
- Replace the sidebar with a horizontal top navigation bar.
- Rebuild the demo trading page: status panel with metric tiles,
  positions table, and a three-column layout (symbols, plan card,
  open orders and timeline); restyle the analysis drawer.
- Fix the stale risk wording in the trading capital dialog.

web/src/main.ts

@@ -3,6 +3,8 @@ import App from "@/App.vue"
 import { router } from "@/router"
 import "element-plus/dist/index.css"
 import "element-plus/theme-chalk/dark/css-vars.css"
+import "@fontsource-variable/inter"
+import "@fontsource-variable/jetbrains-mono"
 import "@/styles/index.css"
 import "virtual:uno.css"