chore: rename module + references to tiennm99/ghstats-cards

Matches the Marketplace name; repo is being renamed in lockstep.

- go.mod module path: github.com/tiennm99/ghstats →
  github.com/tiennm99/ghstats-cards
- Import paths across every .go file updated.
- README badges, install snippets, and the 'go install' line point
  to the new URL/path.
- docs/deployment-guide.md workflow template, Docker image path, and
  release edit URL updated.

Breaking for consumers pinned to the old URL; they need to swap
tiennm99/ghstats → tiennm99/ghstats-cards in workflows and switch
Docker pulls to ghcr.io/tiennm99/ghstats-cards. GitHub's HTTP
redirect covers git clones but GHCR does NOT redirect — users must
update image URIs manually.

internal/card/donut_chart.go

@@ -5,8 +5,8 @@ import (
 	"math"
 	"strings"
 
-	"github.com/tiennm99/ghstats/internal/github"
-	"github.com/tiennm99/ghstats/internal/theme"
+	"github.com/tiennm99/ghstats-cards/internal/github"
+	"github.com/tiennm99/ghstats-cards/internal/theme"
 )
 
 // renderDonutCard draws a donut chart with a left-side legend. Shared by the