fix(studio): 回放视图里选了不是记录的视频时，明确说明原因并给出可行路径

- What: `_load_replay_video()` 先用与 `list_recordings()` 相同的判据确认"这是一条记录"（视频同目录
  必须有 `forces.csv`）；不是记录时不再把该目录当成记录打开，而是给出原因（状态栏 + 2D 面板常驻文案）、
  在 rail 上保留该视频路径，并把「从视频重算力值」置为可用；`_can_recompute()` 统一"重算有没有东西可读"
  （记录或已选视频），`_sync_menu_checks`/`_refresh_replay_state`/`_on_recompute_toggled` 共用它；
  回放按钮在"已选视频但不是记录"时给出对应 tooltip；`_start_playback()` 遇到零长度时间轴时改为明确提示，
  不再静默取消勾选。
- Why: 用户报告"历史回放面板，选择视频后按回放键没有反应"。复现确认：`RecordingReader` 容忍空目录，
  所以随便一段视频所在目录都会被"成功"打开成一条空记录，`duration_s()==0`，按回放→`_start_playback`
  直接静默 return——按钮点了像没点。一条记录的定义（录像 + forces.csv + session.json）此前只写在
  `list_recordings` 里，回放入口没有用同一个判据。
- Verification: 复现脚本三种状态（未选/裸视频/记录内视频）行为符合预期；新增三项 UI 用例
  （裸视频被拒且说明原因并能重算、记录内视频可打开且能播、缺视频段的记录给出提示）；
  apps/studio 全量通过；ui_check 576 项 0 失败（新增 6 项覆盖同一行为）；locale_check 0 缺失。
- Not in this change: 让回放直接播放"非记录"的视频（那需要一条没有力值的纯视频通道；现在用「从视频重算」
  覆盖这个需求）；`RecordingReader` 对空目录的容忍行为（它要允许正在写入、尚未写 session.json 的记录）。

apps/studio/config/locales/en_US.json

@@ -102,6 +102,7 @@
     "playback_tip": "Play this recording at 1× (space bar does the same); dragging the cursor pauses",
     "playback_pause_tip": "Pause playback (dragging the cursor, box zoom or the wheel also pauses)",
     "playback_disabled": "Open a recorded video first (“Choose a recorded video…” at the top left)",
+    "playback_needs_recording": "This video has no recording beside it (no forces.csv): replay needs a recording; to just watch the video use View ▸ Recompute forces from the video",
     "reset": "Reset",
     "units": "Units",
     "units_newton": "Newton",
@@ -205,6 +206,9 @@
     "video_filter": "Recorded video (*.mp4 *.avi *.mov *.mkv);;All files (*)",
     "no_video": "This recording has no usable video segment",
     "open_failed": "Could not open that recording: {error}",
+    "not_a_recording": "“{name}” is not a recording: its folder has no forces.csv (a recording is the video plus forces.csv and session.json). Replay needs a recording; to just watch this video, use “Recompute forces from the video”.",
+    "video_only_panel": "This is not a recording (no forces.csv beside it)\nTo watch this video, use\nView ▸ Recompute forces from the video",
+    "empty_recording": "This recording has no playable picture (no raw_*.mp4 segment), so it cannot be replayed",
     "none_found": "No recording found under outputs/recordings",
     "frame_dropped": "This frame never reached the video (the record queue overflowed)",
     "frame_unreadable": "This frame could not be decoded from the video",

apps/studio/config/locales/zh_CN.json

@@ -102,6 +102,7 @@
     "playback_tip": "按 1× 播放这段录制（空格键同效）；拖动游标即暂停",
     "playback_pause_tip": "暂停播放（拖动游标、框选或滚轮也会暂停）",
     "playback_disabled": "先打开一段录制视频（左上「选择录制视频…」）",
+    "playback_needs_recording": "这段视频旁边没有记录（缺 forces.csv）：回放需要一条记录；只想看视频请用 视图 ▸ 从视频重算力值",
     "reset": "重置",
     "units": "单位",
     "units_newton": "牛顿",
@@ -205,6 +206,9 @@
     "video_filter": "录制的视频 (*.mp4 *.avi *.mov *.mkv);;所有文件 (*)",
     "no_video": "这段录制没有可用的视频分段",
     "open_failed": "无法打开该录制: {error}",
+    "not_a_recording": "「{name}」不是一条记录：同目录没有 forces.csv（一条记录 = 录像 + forces.csv + session.json）。回放需要记录；只想看这段视频请用「从视频重算力值」。",
+    "video_only_panel": "这不是一条记录（同目录缺少 forces.csv）\n只想看这段视频请用\n视图 ▸ 从视频重算力值",
+    "empty_recording": "这条记录没有可播放的画面（没有 raw_*.mp4 视频分段），因此无法回放",
     "none_found": "outputs/recordings 下没有找到录制",
     "frame_dropped": "该帧未被写入视频（记录队列曾溢出）",
     "frame_unreadable": "该帧无法从视频中解码",