fix(web): explicit keep-alive Off/Keep toggle + roomier custom display form

Two UX fixes on the Virtual displays Configuration card:

- Keep-alive is no longer implicitly "on" by typing in the seconds field. It's an
  explicit two-button toggle — **Off** (tear down at disconnect) vs. **Keep for** [N]
  seconds — and the seconds input only appears when "Keep for" is selected. The
  duration is remembered across toggles, and the help text explains both modes.
- Opened up the cramped custom form: the fields container is `space-y-6` with more
  padding (`p-5`, rounded-lg), each option group is `space-y-2.5`, and the Save button
  sits below a divider — so it reads as sections with room instead of a pressed stack.

web tsc + vite build + biome-lint green; deployed on .21.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/messages/en.json

@@ -60,6 +60,7 @@
 	"display_preset_workstation": "Workstation",
 	"display_keep_alive": "Keep alive after disconnect",
 	"display_keep_alive_off": "Off",
+	"display_keep_alive_keep": "Keep for",
 	"display_keep_alive_seconds": "seconds",
 	"display_topology": "Topology",
 	"display_topology_auto": "Automatic",
@@ -85,7 +86,7 @@
 	"display_custom_desc": "Set every option yourself.",
 	"display_preset_current": "Active",
 	"display_preset_soon": "coming soon",
-	"display_keep_alive_help": "How long a display (and, on gamescope, its game) survives after the client disconnects. 0 = tear down immediately.",
+	"display_keep_alive_help": "Off tears the display down as soon as the client disconnects. Keep it alive (and, on gamescope, its game) so a quick reconnect resumes instantly instead of rebuilding.",
 	"display_topology_help": "What happens to the host's physical monitors while streaming.",
 	"display_conflict": "When another client connects",
 	"display_conflict_help": "What happens if a second client connects while one is already streaming and asks for a different resolution.",