chore(13e6): TEMP power-monitor telemetry headers

To validate the PIN_PWR rail-cut change (e2c9d8f) without a bench
multimeter, have the device report its previous cycle's awake time
and panel-init time on each poll:

  X-Prev-Awake-Ms       — millis() at the moment esp_deep_sleep_start
                          armed, last cycle. Total awake duration
                          since reset, ~5–10 s steady-state.
  X-Prev-Panel-Init-Ms  — duration of epd_init() last cycle. Spikes
                          here would suggest the rail isn't coming
                          back up cleanly after the GPIO-hold release.

Headers are sent only when the cached NVS values are non-zero (skips
the first boot under this firmware). All call sites marked `// TEMP:
power-monitor` for clean removal once the change is validated. Two
new NVS keys (tm_awk, tm_pin) sit alongside the existing ones; mock
Preferences extended with getUInt/putUInt to match.

Server side logs the headers via `device.poll.power_telemetry`
(separate commit in pictureFrame-webApp).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

test/mocks/Preferences.h

@@ -8,6 +8,7 @@ extern int g_prefs_putint_seq;  // sequence position of last putInt call
 
 struct Preferences {
     std::map<std::string, int32_t>  ints;
+    std::map<std::string, uint32_t> uints;
     std::map<std::string, std::string> strings;
     bool _open = false;
 
@@ -26,10 +27,16 @@ struct Preferences {
         g_call_seq++;
     }
 
+    uint32_t getUInt(const char* key, uint32_t def = 0) {
+        auto it = uints.find(key);
+        return it != uints.end() ? it->second : def;
+    }
+    void putUInt(const char* key, uint32_t val) { uints[key] = val; }
+
     String getString(const char* key, const char* def = "") {
         auto it = strings.find(key);
         return it != strings.end() ? String(it->second) : String(def);
     }
     void putString(const char* key, const String& val) { strings[key] = val._s; }
-    void clear() { ints.clear(); strings.clear(); }
+    void clear() { ints.clear(); uints.clear(); strings.clear(); }
 };