feat: scenario picker, flow chaining UX, reliable frequency checks

Scenario picker & completion:
- Login → scenario selection screen with complete chains + individual phases
- Completion screen with "fly again / try opposite / back to scenarios"
- Scenario.airport ('dep'|'arr') drives which airport frequencies to fetch;
  arrival scenarios (vfr-arrival, circuit-landing, taxi-in) use arr ICAO

Backend session integration:
- createSession forwards no_chain; response carries active_flow + session_complete
- Pass all six airport frequency variables to session so every chained flow
  has the real airport values from creation
- fetchAirportFrequencies now runs before session creation so resolved
  frequencies are included in backendVariables

Wrong-frequency check:
- airportFreqMap computed (from airportFrequencies, always up-to-date)
  used as primary source in expectedFrequencyForState — immune to flow
  snapshot switches
- setActiveFlow called when response.active_flow changes so local engine
  cursor moves to the correct flow's states after a chain
- Wrong-freq ATC reply appended to communication log (offSchema entry)

Engine fixes (communicationsEngine.ts):
- patchVariables / patchFlags: write directly to the internal reactive
  store, bypassing readonly(ref) which silently blocked all (vars as any)
  .value[k] = v mutations
- appendLogEntry: push ATC speech (and wrong-freq replies) into comm log
- ATC controller_say_rendered appended to comm log after every transmission

app/composables/useRadioBackend.ts

@@ -10,6 +10,7 @@ export interface RadioSessionResponse {
 export interface RadioTransmitResponse {
     session_id: string
     next_state_id: string
+    active_flow: string
     controller_say_template: string | null
     controller_say_rendered: string | null
     expected_pilot_template: string | null
@@ -19,6 +20,8 @@ export interface RadioTransmitResponse {
     fallback_used: boolean
     fallback_reason: string | null
     auto_advanced_states?: string[]
+    /** True when the full chain is done — show the completion screen. */
+    session_complete?: boolean
 }
 
 export function useRadioBackend() {
@@ -31,10 +34,11 @@ export function useRadioBackend() {
     async function createSession(
         flowSlug: string,
         variables?: Record<string, any>,
+        noChain: boolean = false,
     ): Promise<RadioSessionResponse> {
         return await $fetch<RadioSessionResponse>(`${baseUrl()}/api/radio/session`, {
             method: 'POST',
-            body: { flow_slug: flowSlug, variables: variables ?? null },
+            body: { flow_slug: flowSlug, variables: variables ?? null, no_chain: noChain },
         })
     }