feat: instant tab navigation by moving SiteNav to shared layout

SiteNav now lives in the [id] layout instead of each page, so it stays
mounted during route transitions. Switching between Dashboard, Uptime,
Funnels, and Settings no longer flashes a full-page skeleton.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 - **Beautiful funnel visualization.** Funnel reports now show a smooth, animated funnel shape instead of a plain bar chart. Each step flows into the next with curved segments, hover effects, and labels showing visitor counts and conversion percentages at a glance.
 - **Tidier dashboard layout.** The tab navigation (Dashboard, Uptime, Funnels, Settings) now sits above your site name and controls, keeping the tabs front and center.
+- **Instant tab switching.** Clicking between Dashboard, Uptime, Funnels, and Settings now feels instant — the tab bar stays in place while the page content loads below it, instead of the whole screen flashing with a loading skeleton.
 - **Smoother theme switching.** Toggling between light and dark mode now plays a satisfying circular reveal animation that expands from the toggle button, instead of everything just flipping instantly.
 - **Cleaner site navigation.** Dashboard, Uptime, Funnels, and Settings now use an underline tab bar instead of floating buttons. The active section is highlighted with an orange underline, making it easy to see where you are and switch between views.
 - **Consistent icon style.** All dashboard icons now use a single, unified icon set for a cleaner look across Technology, Locations, Campaigns, and Referrers panels.