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/lib/variableResolver.ts

@@ -5,6 +5,21 @@
  * - [USER_INPUT:prompt] → replaced with variables[prompt]
  * - [SAVE_AS:name] → removed from display
  */
+
+export interface UnresolvedVariable {
+  variableName: string
+  token: string // e.g. "[VAR:server_name]"
+}
+
+export interface ResolveResult {
+  text: string
+  unresolvedVariables: UnresolvedVariable[]
+}
+
+/**
+ * Resolve variables in text, replacing tokens with values.
+ * Missing/empty values are replaced with "N/A".
+ */
 export function resolveVariables(text: string, variables: Record<string, string>): string {
   // Replace [VAR:name] — empty/missing values show "N/A"
   let result = text.replace(/\[VAR:([^\]]+)\]/g, (_, name) => {
@@ -26,6 +41,41 @@ export function resolveVariables(text: string, variables: Record<string, string>
   return result
 }
 
+/**
+ * Resolve variables but also return info about which variables are unresolved.
+ * Unresolved variables are left as their original tokens in the text (not replaced with N/A).
+ */
+export function resolveVariablesWithStatus(text: string, variables: Record<string, string>): ResolveResult {
+  const unresolvedVariables: UnresolvedVariable[] = []
+
+  // Replace [VAR:name] — resolved values get substituted, unresolved stay as tokens
+  let result = text.replace(/\[VAR:([^\]]+)\]/g, (match, name) => {
+    const key = name.trim()
+    const value = variables[key]
+    if (value && value.trim()) {
+      return value
+    }
+    unresolvedVariables.push({ variableName: key, token: match })
+    return match // Keep original token for inline prompt rendering
+  })
+
+  // Replace [USER_INPUT:prompt]
+  result = result.replace(/\[USER_INPUT:([^\]]+)\]/g, (match, prompt) => {
+    const key = prompt.trim()
+    const value = variables[key]
+    if (value && value.trim()) {
+      return value
+    }
+    unresolvedVariables.push({ variableName: key, token: match })
+    return match
+  })
+
+  // Remove [SAVE_AS:name]
+  result = result.replace(/\[SAVE_AS:[^\]]+\]/g, '')
+
+  return { text: result, unresolvedVariables }
+}
+
 /**
  * Extract all [USER_INPUT:prompt] tokens from text.
  * Returns array of prompt strings.