feat(maestro): spawn form back in a modal on web

Draft panes are gone: an empty pane is a placeholder again, and every
spawn affordance opens a dialog around the shared spawn form. Escape,
Cancel, the ✕ and a backdrop click dismiss it; a successful submit puts
the session in a new tab (Spawn similar and the fast-forward fallback
keep placing it beside the pane they acted on).

Removes the per-leaf draft state, its persistence strip, and the form
reuse/teardown machinery the panes needed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

DESIGN.md

@@ -100,10 +100,11 @@ Three surfaces, all DOM built in TS, styled by class:
   no-sessions, no-search-match.
 - **Attach** — header (back, `session <id>`, live/closed/error state pill) over
   a framed dark xterm with a green cursor.
-- **Draft pane** — the spawn form mounted inline in an empty split pane,
-  focusable like any session pane; mono inputs with green focus ring; grouped
-  worktree fieldset. Spawn opens one (reusing an empty focused pane, else
-  splitting), and "Spawn similar" seeds one from a session's dir + command.
+- **Spawn modal** — the spawn form in a centered dialog (a bottom sheet below
+  600px) over a blurred backdrop; mono inputs with green focus ring; grouped
+  worktree fieldset. Escape, Cancel, ✕ and a backdrop click all dismiss it.
+  Submitting gives the new session its own tab; "Spawn similar" seeds the form
+  from a session's dir + command and puts the result beside it.
 
 Interactive elements ship default / hover / focus-visible / active / disabled
 states; the accent green is used only for action, selection, focus, and live
@@ -112,5 +113,5 @@ status — never decoration.
 ## Dev review aids (DEV-only, stripped from production)
 
 `web/src/mock.ts` + query params seed a representative catalog without a running
-daemon: `?mock` (seed sessions), `?mock&spawn` (open a draft pane),
+daemon: `?mock` (seed sessions), `?mock&spawn` (open the spawn modal),
 `?theme=light|dark` (pin a theme). Guarded by `import.meta.env.DEV`.