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