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/user/profile.go

@@ -161,15 +161,9 @@ func prepareUserProfileTabData(ctx *context.Context, profileDbRepo *repo_model.R
 		ctx.Data["Cards"] = following
 		total = int(numFollowing)
 	case "activity":
-		// prepare heatmap data
-		if setting.Service.EnableUserHeatmap {
-			data, err := activities_model.GetUserHeatmapDataByUser(ctx, ctx.ContextUser, ctx.Doer)
-			if err != nil {
-				ctx.ServerError("GetUserHeatmapDataByUser", err)
-				return
-			}
-			ctx.Data["HeatmapData"] = data
-			ctx.Data["HeatmapTotalContributions"] = activities_model.GetTotalContributionsInHeatmap(data)
+		if setting.Service.EnableUserHeatmap && activities_model.ActivityReadable(ctx.ContextUser, ctx.Doer) {
+			ctx.Data["EnableHeatmap"] = true
+			ctx.Data["HeatmapURL"] = ctx.ContextUser.HomeLink() + "/-/heatmap"
 		}
 
 		date := ctx.FormString("date")