feat(ui): Video playground tab + video capability plumbing

New Video tab in the Playground: model selector filtered to
video_generation/image_to_video capabilities, prompt + negative prompt,
size/frames/fps/steps/guidance/seed knobs (0 = model default), optional
conditioning-image upload (image-to-video), elapsed-time spinner for
the minutes-long blocking call, inline <video> playback + download.
Calls POST /v1/videos/sync (multipart); resolution rides as both
width/height and size so either upstream convention honors it.

Config side: "video" joins the valid capability modalities, and
/v1/models maps text->video to video_generation and image->video to
image_to_video, mirroring the image mappings. Declare e.g.:

  videogen-wan22-5b:
    capabilities:
      in: [text, image]
      out: [video]

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

internal/server/api.go

@@ -88,6 +88,12 @@ func renderCapabilities(caps config.ModelCapConfig) (arch map[string]any, capsMa
 		if contains(caps.In, "image") && contains(caps.Out, "image") {
 			capsMap["image_to_image"] = true
 		}
+		if contains(caps.In, "text") && contains(caps.Out, "video") {
+			capsMap["video_generation"] = true
+		}
+		if contains(caps.In, "image") && contains(caps.Out, "video") {
+			capsMap["image_to_video"] = true
+		}
 	}
 
 	if caps.Tools {