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/components/layout/NotificationsPanel.tsx

@@ -24,7 +24,7 @@ export function NotificationsPanel() {
         setSessions(data)
         // Mark as "new" if any session was updated in the last hour
         const oneHourAgo = Date.now() - 3600000
-        setHasNew(data.some(s => new Date(s.started_at).getTime() > oneHourAgo))
+        setHasNew(data.some(s => s.started_at && new Date(s.started_at).getTime() > oneHourAgo))
       })
       .catch(() => {})
   }, [])
@@ -90,7 +90,7 @@ export function NotificationsPanel() {
                     <p className="text-[0.6875rem] text-muted-foreground">
                       {session.completed_at
                         ? `Completed ${timeAgo(session.completed_at)}`
-                        : `Started ${timeAgo(session.started_at)}`}
+                        : session.started_at ? `Started ${timeAgo(session.started_at)}` : 'Not started'}
                       {session.client_name && ` · ${session.client_name}`}
                     </p>
                   </div>