fix: replace hardcoded UI labels with i18n translations

- Add missing i18n keys to common.json, reports.json, dashboard.json,
  tasks.json, git.json (EN + RU)
- Replace all hardcoded labels in LLM report page with .reports.* keys
- Replace hardcoded tab labels (Linked resources, Git history) in
  DashboardDetail page
- Replace hardcoded RU text in DashboardGitManager with .git.* keys
- Remove hardcoded fallback strings from DashboardHeader, DashboardTaskHistory

frontend/src/lib/i18n/locales/en/tasks.json

@@ -69,5 +69,7 @@
   "show_diff": "Show diff",
   "diff_preview": "Diff preview",
   "no_diff_available": "Diff is not available",
-  "summary_link_unavailable": "Deep link or diff is unavailable for this task"
+  "summary_link_unavailable": "Deep link or diff is unavailable for this task",
+  "open_llm_report": "LLM report",
+  "result": "Check"
 }