Preview uncommitted changes in the detail pane

For a dirty submodule, list the changed files so the user can remember
what they were doing there:

  Changes   5 files · 2M 1A 1D 1R · +37 −12
            ──────────────────────────────────
            M  render.go  src/render/  +20 −4  3d ago
            ?  scratch.txt                     5m ago

- Status.Changes: files changed against HEAD (staged and unstaged
  combined) with kind, line counts and modification time; sorted by kind,
  then by size; capped at 200 with totals over all files.
- Git provider: parse `status --porcelain=v2 -z` and one
  `diff HEAD --numstat -z -M`, only for dirty submodules.
- Detail pane: flat list with a colored status letter per file, like IDE
  source-control views; counts and age in compact columns; up to 10 files,
  every kind keeps a line, the rest is summarized as "n more".
- Fixture gains libs/work with every kind of change; mock data extended.
- Docs: F24, change preview in the UX guide, D17.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/tui/theme/issues.go

@@ -25,6 +25,42 @@ func IssueWord(k submodule.IssueKind) string {
 	}
 }
 
+// ChangeLetter is the one-letter label of a changed file.
+func ChangeLetter(k submodule.ChangeKind) string {
+	switch k {
+	case submodule.ChangeConflicted:
+		return "!"
+	case submodule.ChangeModified:
+		return "M"
+	case submodule.ChangeAdded:
+		return "A"
+	case submodule.ChangeRemoved:
+		return "D"
+	case submodule.ChangeRenamed:
+		return "R"
+	default:
+		return "?"
+	}
+}
+
+// ChangeStyle returns base with the color of a change kind.
+func (t Theme) ChangeStyle(k submodule.ChangeKind, base lipgloss.Style) lipgloss.Style {
+	switch k {
+	case submodule.ChangeConflicted:
+		return base.Foreground(t.Danger).Bold(true)
+	case submodule.ChangeModified:
+		return base.Foreground(t.Warn)
+	case submodule.ChangeAdded:
+		return base.Foreground(t.Added)
+	case submodule.ChangeRemoved:
+		return base.Foreground(t.Danger)
+	case submodule.ChangeRenamed:
+		return base.Foreground(t.Primary)
+	default:
+		return base.Foreground(t.Muted)
+	}
+}
+
 // LevelStyle returns base with the text color (and weight) for a level.
 func (t Theme) LevelStyle(l submodule.Level, base lipgloss.Style) lipgloss.Style {
 	switch l {