fix: three real sensor-lifecycle and honesty bugs (verified, not assumed)

From the audit whose verifier agents all died on session limits — so I
checked each claim myself rather than trusting it. One was WRONG and is
left alone; three were real.

REFUTED, deliberately unchanged: "the EMF support check is a false
positive, dead on iOS". The iOS gesture flow is correctly implemented
(EmfSensorListener.needsPermission/requestPermission) and the panel calls
it before start(). Nothing to fix; "fixing" it would have broken working
code.

1. Microphone never released when the panel unmounts mid-getUserMedia.
   `this.stream` is only assigned after the await, so stop() during the
   permission prompt found null and released nothing — then the promise
   resolved, set running = true, and the mic went live *after* teardown,
   staying on for the page's life with the recording indicator lit and an
   orphaned rAF loop burning battery. Fixed with a generation counter that
   makes the await cancellable. Proven: the new test fails without the
   guard and passes with it (verified by reverting it).

2. Same bug class in the RTL-SDR panel: sdrRef.current is assigned after
   requestDevice()+open(), so unmounting during the device picker left the
   dongle claimed AND started a sweep against a dead component — only a
   tab close would free it. Added a mountedRef check, mirroring the guard
   EmfPanel already had.

3. EVP blamed the seeker for refusals that never happened. A bare
   `catch {}` set "you refused the microphone" for every failure, so an
   insecure http:// origin, a machine with no mic, and a mic held by
   another app all told the user to go fix a permission that was never
   denied. Now classified from the DOMException name into four honest
   causes (denied / insecure / absent / busy), each with its own copy and
   a working alternative, in both languages.

375 frontend tests pass; i18n parity gate passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/pages/SeancePage.tsx

@@ -543,7 +543,11 @@ function EvpPanel() {
   const hasListeningTool = user?.unlocks?.includes('listening_tool') ?? false
   const [listening, setListening] = useState(false)
   const [pending, setPending] = useState(false)
-  const [denied, setDenied] = useState(false)
+  // Why the mic isn't open, so the notice can be truthful about the cause
+  // instead of always claiming the seeker refused it.
+  const [micFailure, setMicFailure] = useState<
+    'denied' | 'insecure' | 'absent' | 'busy' | null
+  >(null)
   const [bandDb, setBandDb] = useState<number | null>(null)
   const [nyquist, setNyquist] = useState(24000)
   const [markers, setMarkers] = useState<ScopeAnomaly[]>([])
@@ -569,7 +573,7 @@ function EvpPanel() {
 
   const start = async () => {
     if (pending || listening) return
-    setDenied(false)
+    setMicFailure(null)
     setPending(true)
     const listener = listenerRef.current ?? new EvpListener()
     listenerRef.current = listener
@@ -628,9 +632,26 @@ function EvpPanel() {
         }
         setListening(true)
       }
-    } catch {
+    } catch (err) {
       if (listenerRef.current === listener) listenerRef.current = null
-      setDenied(true)
+      // Blaming the seeker for a refusal that never happened sends them to
+      // fix the wrong thing. getUserMedia's DOMException names distinguish
+      // the real causes, so each one gets truthful, actionable copy:
+      // NotAllowedError is a genuine denial; NotFoundError means there's no
+      // microphone at all; NotReadableError means another app holds it; and
+      // on a bare http:// origin mediaDevices is simply absent.
+      const name = err instanceof DOMException ? err.name : ''
+      const noMediaApi =
+        typeof navigator === 'undefined' || !navigator.mediaDevices?.getUserMedia
+      setMicFailure(
+        noMediaApi || name === 'SecurityError'
+          ? 'insecure'
+          : name === 'NotFoundError' || name === 'OverconstrainedError'
+            ? 'absent'
+            : name === 'NotReadableError' || name === 'AbortError'
+              ? 'busy'
+              : 'denied',
+      )
       setBandDb(null)
     } finally {
       setPending(false)
@@ -677,10 +698,10 @@ function EvpPanel() {
           {t('seance.evp.listening')}
         </p>
       )}
-      {denied && (
+      {micFailure && (
         <div className="panel-notice error">
-          <h4>{t('seance.evp.deniedTitle')}</h4>
-          <p>{t('seance.evp.denied')}</p>
+          <h4>{t(`seance.evp.fail.${micFailure}Title`)}</h4>
+          <p>{t(`seance.evp.fail.${micFailure}`)}</p>
         </div>
       )}
       <button
@@ -716,9 +737,15 @@ function RadioPanel() {
   const lastTuneUiRef = useRef(0)
   const sonifierRef = useRef(new SpectrumSonifier())
 
+  // False once this panel unmounts, so an open sequence still awaiting the
+  // device picker can tell it was abandoned — sdrRef alone can't cover that
+  // window, because it isn't assigned until after the awaits complete.
+  const mountedRef = useRef(true)
+
   // Power down the dongle on unmount (mode switch / leaving the page).
   useEffect(
     () => () => {
+      mountedRef.current = false
       const sdr = sdrRef.current
       sdrRef.current = null
       detectorRef.current = null
@@ -769,6 +796,15 @@ function RadioPanel() {
       void sdr.close()
       return
     }
+    // requestDevice() waits on the browser's device picker and open() runs
+    // ~20 sequential control transfers, so the seeker has ample time to
+    // switch modes or leave. Without this the dongle would be left claimed
+    // and sweeping forever against an unmounted panel, and could only be
+    // freed by closing the tab.
+    if (!mountedRef.current) {
+      void sdr.close()
+      return
+    }
     sdrRef.current = sdr
     detectorRef.current = new SpectrumAnomalyDetector()
     setPhase('sweeping')