feat(studio): 回放可以直接播放「一条视频」——不必先按「从视频重算」

- What: 新增 `record/playback.py::VideoOnlySource`（一个视频文件的时间轴：时长来自
  `CAP_PROP_FRAME_COUNT/FPS`，按位置解码、顺序前进时只 step 不 seek、越界钳到最后一帧；没有帧计数或帧率就
  拒绝而不是猜）；`ReplayController` 增加 `open_video()`/`has_video()`/`is_video_only()`/`video_path()`，
  `duration_s`/`set_position`/`_refresh_video`/`close` 都覆盖这条路径，并在 `warnings_text()` 里说明
  "这段视频没有配套记录"；`app_window._load_replay_video()` 改为按"这条视频是什么"处理：
  同目录有 `forces.csv` → 当记录打开（原行为），否则**当视频播放**（曲线为空、明确提示），只有连解码都不行
  才拒绝并给出 `从视频重算` 的出路；transport/`_render_replay_frame`/`_replay_owns_image`/状态栏 tooltip
  一律改用 `has_video()`（而查曲线的 `_apply_replay_source` 仍用 `is_loaded()`）。
- Why: 用户要求"只导入视频也能用回放按钮"。此前有录像必须走「从视频重算」才能动，因为一条裸视频被当成
  "不是记录"而拒绝（更早是当成空记录"成功"打开→零长度时间轴→按回放没反应）。这两种都不对：视频就是可以看的
  东西，只是没有力值；把它按视频播放、把"没有记录"如实写在标签与提示里，才既不骗人也不挡人。
- Verification: `shared/tests/test_playback.py` +3（VideoOnlySource：时长/按位置取帧/顺序前进/越界钳位/关闭后
  返回空/不可解码与无时间轴被拒；顺带修回我在插入新类时弄丢的 `list_recordings` 的 `base = Path(root)`）；
  `test_ui_smoke.py` 改 1 增 1（裸视频可播放：`is_video_only`、transport 可用、tooltip 说明曲线为空、
  按下后位置推进且解出画面、停止有效；不可解码文件仍被拒并提示）；ui_check 587 项 0 失败（回放段改为验证
  "裸视频能播"）；apps/studio 与 shared 全量通过；locale_check 0 缺失。
- Not in this change: 裸视频的曲线（没有 CSV 就没有力值，这是事实而不是待办）；裸视频的三维形变（要形变得走
  「从视频重算」，面板标签在回放里仍然是"录像不含三维场"）；多段视频拼接播放。

apps/studio/config/locales/en_US.json

@@ -103,6 +103,7 @@
     "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",
+    "playback_video_only": "Play this video (no recording beside it: the picture plays, the force charts are empty)",
     "reset": "Reset",
     "stop_playback": "Stop",
     "stop_playback_tip": "Stop the replay: plain playback and a “recompute from the video” pass both stop here (Esc does the same).",
@@ -213,7 +214,9 @@
     "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",
+    "video_only_opened": "“{name}” has no recording beside it (no forces.csv), so it is playing as a video: the picture and the timeline work, the force charts are empty. For forces and deformation use View ▸ Recompute forces from the video.",
+    "video_only_notice": "This video has no recording beside it (no forces.csv): it plays, but the force charts and the recording details are empty.",
+    "video_only_panel": "This is not a recording, and this video cannot be decoded (no forces.csv beside it, and it does not read as playable video)\nYou can still try View ▸ 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)",

apps/studio/config/locales/zh_CN.json

@@ -103,6 +103,7 @@
     "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",
+    "playback_video_only": "播放这段视频（没有配套记录：可以看画面，力曲线为空）",
     "reset": "重置",
     "stop_playback": "停止",
     "stop_playback_tip": "停止回放：无论是普通播放还是「从视频重算」通道，都在这里停下（Esc 同效）。",
@@ -213,7 +214,9 @@
     "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",
+    "video_only_opened": "「{name}」没有配套记录（无 forces.csv），已按视频播放：可以看画面与拖时间轴，力曲线为空。要得助力值/形变请用 视图 ▸ 从视频重算力值。",
+    "video_only_notice": "这段视频没有配套记录（同目录无 forces.csv）：可以播放，但力曲线与记录信息为空。",
+    "video_only_panel": "这不是一条记录，而且无法解码这段视频（没有 forces.csv，也不像可播放的视频文件）\n可以试试 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）",