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>

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

frontend/src/pages/MyTreesPage.tsx

@@ -56,7 +56,7 @@ export function MyTreesPage() {
       const lastUsedMap = new Map<string, string>()
       for (const session of recentSessions) {
         const existing = lastUsedMap.get(session.tree_id)
-        if (!existing || new Date(session.started_at) > new Date(existing)) {
+        if (session.started_at && (!existing || new Date(session.started_at) > new Date(existing))) {
           lastUsedMap.set(session.tree_id, session.started_at)
         }
       }