docs(threading): add VHS demo for #509

threading_demo.tape captures: flat (5 emails) -> threaded (3 rows with
'(3)' count on the root) -> expanded thread (5 rows with indented '↪'
markers on the children) -> collapsed -> back to flat. Rendered to
public/assets/threading_demo.gif.

Author: mvanhorn

public/assets/threading_demo.gif

@@ -1,27 +1,44 @@
-# Screenshot: Threaded Conversation View
-# Shows flat inbox, threaded collapsed root, then expanded thread tree
+# VHS demo for threaded conversation view (#509)
+# Shows: flat inbox (5 messages) -> threaded view (3 rows, thread root with count) -> expanded thread tree
 
-Output screenshots/threading_demo.gif
+Output threading_demo.gif
 
 Set FontSize 14
 Set FontFamily "JetBrainsMono Nerd Font"
 Set Width 1400
-Set Height 800
+Set Height 900
 Set Theme "Catppuccin Mocha"
 Set Padding 20
+Set Framerate 30
+Set PlaybackSpeed 0.7
+
 Set WindowBar Colorful
 Set WindowBarSize 40
 Set BorderRadius 10
 
-Hide
-Type "go run ./screenshots/cmd/threading_demo"
+Sleep 500ms
+Type "./bin/threading_demo"
+Sleep 300ms
 Enter
-Show
 
-Sleep 1s
+# Boot - flat view, 5 emails listed
+Sleep 4s
+
+# Toggle to threaded - now 3 rows (thread root collapses 3 messages)
 Type "T"
-Sleep 1s
+Sleep 2500ms
+
+# Cursor is on row 1 which is the thread root
+# Enter expands it
 Enter
-Sleep 1s
+Sleep 3000ms
+
+# Enter again to collapse
+Enter
+Sleep 2000ms
+
+# Toggle back to flat
+Type "T"
+Sleep 2000ms
 
-Screenshot screenshots/threading_demo.png
+Ctrl+c