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/package.json

@@ -10,6 +10,8 @@
   },
   "dependencies": {
     "@element-plus/icons-vue": "^2.3.2",
+    "@fontsource-variable/inter": "^5.3.0",
+    "@fontsource-variable/jetbrains-mono": "^5.3.0",
     "axios": "^1.20.0",
     "dayjs": "^1.11.23",
     "element-plus": "^2.14.7",