feat(vocab): update course today plan localization and UI logic

- Added new localization strings for scenarios with no due reviews in German, English, and Spanish, enhancing user guidance.
- Updated VocabCourseView to conditionally display the appropriate introduction message based on the presence of due reviews, improving clarity in the course flow.

frontend/src/i18n/locales/en/socialnetwork.json

@@ -670,8 +670,18 @@
                 "courseFlowEyebrow": "Daily flow",
                 "courseFlowTitle": "Best next step for today",
                 "courseFlowIntro": "Start with today’s suggested order at the top. Below that, the four areas in detail: due quick reviews, current block, intensive review, free practice.",
+                "quickReviewTitle": "Quick Review",
+                "quickReviewIntro": "Short session with {count} terms. After completion, the scheduled review is marked as done.",
+                "quickReviewDoneTitle": "Done",
+                "quickReviewDoneScore": "Correct: {correct} / {total}",
+                "quickReviewBackToCourse": "Back to course",
+                "quickReviewProgress": "Term {current} of {total}",
+                "quickReviewPromptMeaning": "What does \"{term}\" mean?",
+                "quickReviewPromptTarget": "Type in target language: \"{term}\"",
+                "quickReviewAcknowledge": "Read, continue",
                 "courseTodayPlanTitle": "Suggested for today",
                 "courseTodayPlanIntro": "Practical order: first everything that is due for a quick review, then open lessons in your current block, then intensive review if shown. Quick reviews are three short revisit dates after you finish a lesson (typically about 1, 3 and 7 days apart) so vocabulary sticks.",
+                "courseTodayPlanIntroNoDueReview": "No quick review is due today. Start with the open lessons in your current block, then do intensive review if shown. Quick reviews reappear automatically on the 1/3/7-day rhythm.",
                 "courseTodayPlanStepReviewDue": "Quick review now",
                 "courseTodayPlanStepBlock": "Continue in your current block",
                 "courseTodayPlanStepIntensive": "Intensive review",