Use hardware RNG for session passkey and PKC extra nonce (#11047)

* Use hardware RNG for session passkey and PKC extra nonce

The admin session passkey and the Curve25519 extra nonce were drawn
from Arduino random(), which is not a CSPRNG. Source them from
HardwareRNG::fill, mirroring the signing path, and fall back to the
seeded CSPRNG (CryptRNG) only when no hardware source is available.

* AdminModule: make session passkey expiry rollover-safe

session_time was compared as millis()/1000 seconds with additive
thresholds, which breaks across the millis() wrap and could keep a stale
admin session key valid. Store session_time in millis() and use
Throttle::isWithinTimespanMs for the 150s refresh and 300s validity
windows.

* AdminModule: track session passkey validity with an explicit flag

session_time == 0 was used as the uninitialized sentinel, but millis()
is legitimately 0 in the first millisecond of uptime, so a passkey
issued then would be treated as no session. Use a dedicated
session_passkey_valid flag instead.

* AdminModule: camelCase the session passkey validity flag

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/modules/AdminModule.h

@@ -41,7 +41,8 @@ class AdminModule : public ProtobufModule<meshtastic_AdminMessage>, public Obser
     bool hasOpenEditTransaction = false;
 
     uint8_t session_passkey[8] = {0};
-    uint session_time = 0;
+    uint32_t session_time = 0;        // millis() when the current session passkey was issued
+    bool sessionPasskeyValid = false; // separate flag: millis() 0 at boot is a valid issue time
 
     void saveChanges(int saveWhat, bool shouldReboot = true);