feat(web): new task opens straight into chat, context rail waits for content, frosted windows

New Task now opens directly as an empty ChatThread (NewTaskChat) instead of
a separate compose screen, sized like a real task window. The Scope/Activity
context rail in a task window no longer renders until there's actually
something to show (touched entities, activity, or an open question),
avoiding an empty-placeholder sidebar on every new task. Also fixes the
chat input defaulting to several lines tall on window open, centers the
empty-chat greeting vertically, and gives floating windows the same
frosted-glass look as the desktop's task launcher card.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/app.css

@@ -254,7 +254,12 @@
   flex-direction: column;
   box-sizing: border-box;
   pointer-events: auto;
-  background: var(--card);
+  /* Frosted glass — same recipe as the desktop's "What should Nomos do?"
+     launcher card (bg-card/70 backdrop-blur), so floating windows read as
+     part of the same surface language instead of opaque panels. */
+  background: color-mix(in oklab, var(--card) 70%, transparent);
+  backdrop-filter: blur(8px);
+  -webkit-backdrop-filter: blur(8px);
   color: var(--card-foreground);
   border: 1px solid var(--border);
   border-radius: var(--radius-lg);