fix: modal scroll, SSH key textarea, nav full-page, remove OS/Provider

- Fix modal scroll: restructure overlay with scroll/centering wrapper
- Add SSH key textarea + file upload to host modal (authType='key')
- Fix textarea reset on new host (clear via DOM)
- SSH username placeholder instead of default 'root'
- Nav uses plain <a href> instead of HTMX for full-page navigation
- /hosts route returns full page; /hosts/list for HTMX partials
- Remove OS and Provider from host modal, grid card, and list card
- Fix key modal x-data scope for Alpine $dispatch
- Update key_grid to display multi-line key content properly
- Simplify SSH key placeholder format

app/backend/main.go

@@ -61,7 +61,8 @@ func main() {
 	app.Static("/static", "./static")
 
 	app.Get("/", handler.Dashboard)
-	app.Get("/hosts", handler.DashboardList)
+	app.Get("/hosts", handler.Dashboard)
+	app.Get("/hosts/list", handler.DashboardList)
 	app.Post("/hosts", handler.CreateHost)
 	app.Put("/hosts/:id", handler.UpdateHost)
 	app.Delete("/hosts/:id", handler.DeleteHost)