chore: add TUI screenshots via VHS

Screenshots capture 4 views: monitors dashboard (hero), detail panel,
alerts tab, and logs tab. Includes VHS tape, demo seed config, and
setup script for reproducible captures.

Also fixes latencySparkline to color DOWN checks red instead of green
— previously failed checks with 0ms latency rendered as green bars.

vhs/demo.tape

@@ -0,0 +1,54 @@
+Set Shell "bash"
+Set Width 1400
+Set Height 800
+Set FontSize 14
+Set Padding 20
+Set Framerate 15
+Set TypingSpeed 50ms
+
+Hide
+Type "bash vhs/setup.sh /tmp/uptop-vhs.db"
+Enter
+Sleep 45s
+Show
+Sleep 5s
+
+# Sites tab — hero shot with mixed monitor states
+Screenshot vhs/screenshots/monitors.png
+Sleep 1s
+
+# Navigate to Nextcloud (row 6: group + 3 children + Auth Portal)
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Type "i"
+Sleep 3s
+Screenshot vhs/screenshots/detail.png
+Sleep 1s
+
+# Close detail
+Escape
+Sleep 1s
+
+# Tab to Alerts
+Tab
+Sleep 2s
+Screenshot vhs/screenshots/alerts.png
+Sleep 1s
+
+# Tab to Logs
+Tab
+Sleep 2s
+Screenshot vhs/screenshots/logs.png
+Sleep 1s
+
+# Quit
+Type "q"
+Sleep 1s