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/TreeNavigationPage.tsx

@@ -177,10 +177,10 @@ export function TreeNavigationPage() {
 
   const deriveCurrentStepEnteredAt = (sessionData: Session): string => {
     if (!sessionData.decisions || sessionData.decisions.length === 0) {
-      return sessionData.started_at
+      return sessionData.started_at || new Date().toISOString()
     }
     const lastDecision = sessionData.decisions[sessionData.decisions.length - 1]
-    return lastDecision.exited_at || lastDecision.timestamp || sessionData.started_at
+    return lastDecision.exited_at || lastDecision.timestamp || sessionData.started_at || new Date().toISOString()
   }
 
   const openCompletionModal = (completionDecision: DecisionRecord, source: CompletionSource) => {
@@ -320,7 +320,7 @@ export function TreeNavigationPage() {
         client_name: clientName || undefined,
       })
       setSession(newSession)
-      setCurrentStepEnteredAt(newSession.started_at)
+      setCurrentStepEnteredAt(newSession.started_at || new Date().toISOString())
       setShowMetadataForm(false)
       // Save for "Repeat Last Session"
       safeSetItem('last-session', JSON.stringify({