Add peer-to-peer file drop

Files move directly between peers over the overlay, with no server in the
path. The receiver listens on the WireGuard address only, so the port is
unreachable from outside the tunnel, and every offer is matched to a known
peer before anything is read.

Consent is the default: an offer carries metadata alone, and no payload
moves until the receiver accepts. Policy is per profile and device-local —
off, ask, or auto-accept, with per-sender exceptions on top.

Policy and history live in the profile's preferences, so removing a profile
takes its file drop state with it. Transfers interrupted by a restart are
settled on load; nothing survives to finish them, and left alone they would
sit in the log as permanently pending.

The Android bindings pull payload bytes through a chunk-returning stream:
gomobile copies a []byte argument into a fresh Java array and never copies
it back, so a fill-my-buffer method would hand back the right length with
no data.

client/android/profile_state.go

@@ -48,6 +48,33 @@ func profileAccountPathFor(configPath string) (string, error) {
 	return filepath.Join(filepath.Dir(configPath), stem+profileAccountSuffix), nil
 }
 
+// profileLocationFor splits a profile's config path back into the config dir and
+// the profile ID: <dir>/netbird.cfg is the default profile, while
+// <dir>/profiles/<id>.json is a named one.
+func profileLocationFor(configPath string) (string, string, error) {
+	if configPath == "" {
+		return "", "", fmt.Errorf("empty config path")
+	}
+
+	base := filepath.Base(configPath)
+	dir := filepath.Dir(configPath)
+
+	if base == defaultConfigFilename {
+		return dir, profilemanager.DefaultProfileName, nil
+	}
+
+	if filepath.Base(dir) != profilesSubdir {
+		return "", "", fmt.Errorf("config path %q is outside the profiles directory", configPath)
+	}
+
+	id := strings.TrimSuffix(base, filepath.Ext(base))
+	if !profilemanager.IsValidProfileFilenameStem(profilemanager.ID(id)) {
+		return "", "", fmt.Errorf("config path %q has no valid profile ID", configPath)
+	}
+
+	return filepath.Dir(dir), id, nil
+}
+
 // readProfileEmail returns the account email stored for the profile whose config
 // lives at configPath. A missing or unreadable file yields "", which leaves the
 // account choice to the IdP.