chore(claude-do): fix(ui): DiffModal — Commit-Range ohne HeadCommit zeigt stil

Befund (bestätigt): src/ClaudeDo.Ui/ViewModels/Modals/DiffModalViewModel.cs, LoadAsync (~Zeile 116): bei FromCommitRange=true aber HeadCommit==null fällt der Ternary still auf GetBranchDiffAsync(WorktreePath, BaseRef) zurück. In diesem Modus ist WorktreePath aber das Listen-Working-Dir (Repo-Root, kein Worktree) — es wird ein falscher Diff angezeigt, ohne jeden Hinweis.

Änderungen:
1. Guard: From

ClaudeDo-Task: d667c80c-3f32-478c-8584-46aec78357b6

src/ClaudeDo.Localization/locales/en.json

@@ -422,7 +422,7 @@
     "planningBadge": { "active": "PLANNING", "finalized": "PLANNED" },
     "taskRow": { "createdPrefix": "Created {0}", "stepsText": "{0}/{1} steps" },
     "tasksIsland": { "completedHeader": "COMPLETED", "completedHeaderCount": "COMPLETED · {0}" },
-    "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show." },
+    "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show.", "unavailable": "Diff no longer available — commit range incomplete." },
     "planningDiff": { "hubError": "Could not build combined preview (hub error).", "conflict": "Cannot build combined preview: subtask {0} conflicts with an earlier subtask ({1} files)." },
     "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
     "conflictResolution": { "vsCodeError": "Could not launch VS Code: {0}. Paths are listed above — copy them manually.", "subtaskPrefix": "Conflicts in subtask: {0}", "targetPrefix": "Merging into: {0}" },