refactor: split language card into repos-per-language and most-commit-language

Align card set with github-profile-summary-cards' 5-card layout:

  0-profile-details.svg       (unchanged)
  1-repos-per-language.svg    (new) owned repos grouped by primary language
  2-most-commit-language.svg  (new) last-year commits attributed to each repo's primary language
  3-stats.svg                 (renumbered)
  4-productive-time.svg       (renumbered)

- FetchProductive now fills p.CommitsByLanguage from the same commit history
  it uses for the heatmap, so no extra API calls are introduced.
- TopRepos carries primary language so productive-time can aggregate by lang.
- LangStat.Bytes renamed to Value (repo count or commit count, context-dependent).
- Shared bar+legend renderer extracted to language_bar.go.
- Ignore generated output/ directory.

internal/card/most_commit_language.go

@@ -0,0 +1,14 @@
+package card
+
+import (
+	"github.com/tiennm99/ghstats/internal/github"
+	"github.com/tiennm99/ghstats/internal/theme"
+)
+
+type mostCommitLanguageCard struct{}
+
+func (mostCommitLanguageCard) Filename() string { return "2-most-commit-language.svg" }
+
+func (mostCommitLanguageCard) SVG(p *github.Profile, t theme.Theme) ([]byte, error) {
+	return renderLanguageCard("Most Commit Language (last year)", p.CommitsByLanguage, t), nil
+}