Load heatmap data asynchronously (#36622)

Fixes: https://github.com/go-gitea/gitea/issues/21045

- Move heatmap data loading from synchronous server-side rendering to
async client-side fetch via dedicated JSON endpoints
- Dashboard and user profile pages no longer block on the expensive
heatmap DB query during HTML generation
- Use compact `[[timestamp,count]]` JSON format instead of
`[{"timestamp":N,"contributions":N}]` to reduce payload size
- Public API (`/api/v1/users/{username}/heatmap`) remains unchanged
- Heatmap rendering is unchanged, still shows a spinner as before, which
will now spin a litte bit longer.

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/web.go

@@ -888,6 +888,8 @@ func registerWebRoutes(m *web.Router) {
 		m.Group("/{org}", func() {
 			m.Get("/dashboard", user.Dashboard)
 			m.Get("/dashboard/{team}", user.Dashboard)
+			m.Get("/dashboard/-/heatmap", user.DashboardHeatmap)
+			m.Get("/dashboard/-/heatmap/{team}", user.DashboardHeatmap)
 			m.Get("/issues", user.Issues)
 			m.Get("/issues/{team}", user.Issues)
 			m.Get("/pulls", user.Pulls)
@@ -1024,6 +1026,7 @@ func registerWebRoutes(m *web.Router) {
 		}
 
 		m.Get("/repositories", org.Repositories)
+		m.Get("/heatmap", user.DashboardHeatmap)
 
 		m.Group("/projects", func() {
 			m.Group("", func() {