feat(ui): richer diff viewer + surface child roadblocks on parents

- UnifiedDiffParser detects added/deleted/renamed/binary files; diff
  modal shows a file list, binary/empty placeholders, and can diff a
  merged task by commit range after its worktree is gone
- DetailsIslandViewModel flags children needing attention (failed,
  cancelled, awaiting review, or with roadblocks) on the parent
- GitService gains worktree head-commit/range support; planning chain,
  merge orchestration, and session manager tweaks with updated tests
- refresh app/installer/worker icons

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Localization/locales/de.json

@@ -238,7 +238,10 @@
     "diff": {
       "title": "DIFF",
       "windowTitle": "Diff",
-      "merge": "Mergen…"
+      "merge": "Mergen…",
+      "filesHeader": "Dateien",
+      "binary": "Binärdatei — kein Text-Diff",
+      "empty": "Kein Inhalt"
     },
     "worktree": {
       "title": "Worktree"

src/ClaudeDo.Localization/locales/en.json

@@ -238,7 +238,10 @@
     "diff": {
       "title": "DIFF",
       "windowTitle": "Diff",
-      "merge": "Merge…"
+      "merge": "Merge…",
+      "filesHeader": "Files",
+      "binary": "Binary file — no text diff",
+      "empty": "No content"
     },
     "worktree": {
       "title": "Worktree"