feat: flexible intake — deferred variables + prepared sessions (#103)

* feat: flexible intake — deferred variables + prepared sessions

Remove blocking intake form modal. Variables are now filled inline during
flow execution or pre-filled via prepared sessions. Adds PATCH /sessions/{id}/variables
endpoint, POST /sessions/prepare for session pre-staging, inline variable prompts
in StepDetail, editable Session Variables panel, and "Prepared for You" dashboard section.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: pass treeData directly to startSession to avoid stale state

setTree(treeData) hasn't committed when startSession runs immediately
after, so tree is still null and getStepsFromTree returns []. This
caused the step detail area to render empty on new session start.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: wire PrepareSessionModal entry point in Flow Library

Add "Prepare session" button (clipboard icon) to grid, list, and table
views for procedural/maintenance flows. Clicking fetches tree intake
fields and account members, then opens PrepareSessionModal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SessionDetailPage.tsx

@@ -293,7 +293,7 @@ export function SessionDetailPage() {
 
   const getTotalDuration = () => {
     if (!session?.completed_at) return 'In progress'
-    const startedAtMs = new Date(session.started_at).getTime()
+    const startedAtMs = new Date(session.started_at || Date.now()).getTime()
     const completedAtMs = new Date(session.completed_at).getTime()
     if (Number.isNaN(startedAtMs) || Number.isNaN(completedAtMs)) return 'Unknown'
     const seconds = Math.max(0, Math.floor((completedAtMs - startedAtMs) / 1000))
@@ -358,7 +358,7 @@ export function SessionDetailPage() {
           <p className="mt-1 text-sm text-muted-foreground">
             {session.tree_snapshot?.name}
             {session.client_name && <> · Client: {session.client_name}</>}
-            {' · '}{new Date(session.started_at).toLocaleDateString(undefined, { month: 'short', day: 'numeric', year: 'numeric' })}
+            {session.started_at && <>{' · '}{new Date(session.started_at).toLocaleDateString(undefined, { month: 'short', day: 'numeric', year: 'numeric' })}</>}
           </p>
         </div>
         <ActionMenu
@@ -482,7 +482,7 @@ export function SessionDetailPage() {
       <SessionTimeline
         decisions={session.decisions}
         treeType={(session.tree_snapshot as unknown as Record<string, unknown>).tree_type as string}
-        startedAt={session.started_at}
+        startedAt={session.started_at || ''}
         completedAt={session.completed_at}
       />