feat: batch video upscaling with folder upload, directory save, and result comparison (#1051)

* feat: support batch video upscaling with folder upload and directory save

- Add multi-file selection and entire folder upload (webkitdirectory)
- Support drag & drop of both files and folders via FileSystemEntry API
- Add save directory picker using File System Access API (showDirectoryPicker)
- Process videos sequentially with per-file progress and status tracking
- Add right-side result list with click-to-compare functionality
- Add per-video duration and total batch duration display
- Add retry button for failed items with error detail display
- Remove auto-play on comparison viewer load
- Fix worker error handling: handle "error" command to reject stale promises
- Fix event listener accumulation in video comparison (AbortController)
- Fix XSS risk by escaping HTML in all innerHTML injections
- Fix URL revocation timing in download to prevent large file truncation
- Extract writeToSaveDirectory and truncateName helpers to reduce duplication
- Add Usage Guide panel explaining single/multiple/folder upload options

Files changed:
  docs/upscaler/index.html  (+1110/-181)
  docs/upscaler/upscaler.js (+110)

* fix: prevent auto-play and resource consumption during upscaling

- Set preload="metadata" on preview and comparison video elements
- Explicitly pause preview video after setting src in showVideoPreview
- Remove preview video seek sync from batch processing progress callbacks
  (was causing continuous decode during upscaling, wasting GPU resources)
- Remove preview video seek sync from retry progress callback
- All videos now require manual play; no background resource usage

* fix: improve upscaled video quality with higher bitrate and codec support

- Raise base bitrate from 2.5 Mbps (25e5) to 8 Mbps (8e6) for standard and
  firefox processors, preventing quality loss when source videos have high
  bitrates (e.g. 720p @ 16.7 Mbps was producing 1440p @ 10 Mbps, smaller
  than the original file)
- Double the bitrate for mediabunny processor (X.n1 * 2) to account for the
  4x pixel increase from 2x upscaling
- Add H.264 Level 5.1 (avc1.640033) and Level 5.2 (avc1.640034) codecs to
  the fallback lists, since Level 4.0 (avc1.640028, ~24.6 Mbps) cannot
  support the higher bitrates for 1440p+ outputs
- Add latencyMode: "quality" to VideoEncoder configs for both standard and
  firefox processors, prioritizing encoding quality over latency (suitable
  for offline processing)

* feat: add video data comparison panel for original vs upscaled metrics

- Add a comparison data table below the file info bar showing 6 metrics:
  Resolution, File Size, Duration, Est. Bitrate, Pixels, Process Time
- Display per-metric change percentage with color-coded indicators:
  green (↑) for improvements, red (↓) for regressions, gray for neutral
- Estimated bitrate calculated from file size × 8 / video duration
- Table renders after both original and upscaled video metadata loads
- Auto-hide panel on reset

* fix: make video data comparison panel full-width and responsive

- Move comparison data table from inside the left column to the full-width
  area below the left/right layout, so it is no longer constrained by the
  comparison viewer column width
- Remove table-fixed colgroup and overflow-x-auto that caused horizontal
  scrolling on smaller screens
- Table now uses w-full auto-layout, displaying all 4 columns (Metric,
  Original, Upscaled, Change) without requiring horizontal scrolling

---------

Co-authored-by: rionzhao <rionzhao@tencent.com>

assets/sample/subtitle.vtt

@@ -0,0 +1,171 @@
+WEBVTT
+
+1
+00:00:00.306 --> 00:00:00.844
+那一天
+あの日
+
+2
+00:00:01.280 --> 00:00:02.220
+星星降落的日子
+星が降った日
+
+3
+00:00:03.148 --> 00:00:04.412
+那简直是
+それはまるで
+
+4
+00:00:04.446 --> 00:00:04.966
+简直
+まるで
+
+5
+00:00:05.120 --> 00:00:06.300
+像梦中的景色一样
+夢の景色のように
+
+6
+00:00:07.438 --> 00:00:08.978
+美丽的景色
+美しい眺めだった
+
+7
+00:00:24.596 --> 00:00:25.089
+放学后
+放課後
+
+8
+00:00:25.705 --> 00:00:26.261
+不好
+悪い
+
+9
+00:00:26.475 --> 00:00:27.679
+我今天开始打工
+おれ今日これからバイト
+
+10
+00:00:29.345 --> 00:00:31.689
+已经讨厌这样的乡下了
+もうこんな田舎嫌だよー
+
+11
+00:00:31.969 --> 00:00:33.527
+讨厌这样的人生
+こんな人生嫌だ
+
+12
+00:00:33.927 --> 00:00:37.970
+来世请让我成为东京的帅哥吧
+来世は東京のイケメン男子にしてくださーい
+
+13
+00:00:42.083 --> 00:00:42.874
+哪里
+どこ
+
+14
+00:00:43.052 --> 00:00:44.166
+那么说来
+そういえば
+
+15
+00:00:44.554 --> 00:00:47.078
+我觉得好像一直做着奇怪的梦
+ずっと変な夢を見てたような気がするんだけど
+
+16
+00:00:48.324 --> 00:00:50.626
+别人的人生中…梦
+別の人の人生の…夢
+
+17
+00:00:51.452 --> 00:00:52.740
+这是什么
+何だ これ
+
+18
+00:00:54.778 --> 00:00:55.442
+这个，这个
+これって
+
+19
+00:00:56.724 --> 00:00:57.724
+这难道是
+これってもしかして
+
+20
+00:00:58.076 --> 00:00:59.328
+我们在梦中
+俺達は夢の中で
+
+21
+00:01:00.136 --> 00:01:01.202
+替换着
+入れ替わってる
+
+22
+00:01:07.369 --> 00:01:08.297
+确实的事情
+確かなことが
+
+23
+00:01:08.757 --> 00:01:09.541
+只有一个
+一つだけある
+
+24
+00:01:10.325 --> 00:01:11.037
+我们是
+私達は
+
+25
+00:01:11.425 --> 00:01:12.317
+见面的话绝对
+会えば絶対
+
+26
+00:01:12.681 --> 00:01:13.531
+马上就明白
+すぐにわかる
+
+27
+00:01:14.255 --> 00:01:15.279
+我想说
+言おうと思ったんだ
+
+28
+00:01:16.075 --> 00:01:17.579
+你是世界的无论在哪里
+お前が世界のどこにいても
+
+29
+00:01:17.955 --> 00:01:19.051
+一定要去见你
+必ず会いに行くって
+
+30
+00:01:20.141 --> 00:01:20.943
+你是
+あなたは
+
+31
+00:01:21.361 --> 00:01:21.803
+谁
+誰
+
+32
+00:01:22.197 --> 00:01:22.759
+你是
+お前は
+
+33
+00:01:23.483 --> 00:01:24.039
+谁
+誰だ
+
+34
+00:01:25.693 --> 00:01:26.465
+你的名字是
+君の名は