feat: user path to resume BT from low-memory pause via reader menu toggle

Field session: after a recovery teardown the heap settles at ~72 KB --
below the 100 KB start floor -- and never recovers on its own (the
defrag silent-restart only fires when a build FAILS, and builds succeed
now). BT stayed paused forever, the menu toggle claimed ON, and toggling
off/on changed nothing.

- Menu label tells the truth: ON / PAUSED (enabled but stack down) / OFF
  (new STR_STATE_PAUSED string)
- Toggling BT on below the heap floor silent-restarts into the current
  book: the fresh boot's ~118 KB passes the gate and BT auto-starts on
  resume. Explicit user intent is the right trigger for the defrag.
- Hoist the floor to bleinput::kStartMinFreeHeap, shared by the
  lifecycle gate and the toggle

src/BleInput.h

@@ -25,6 +25,12 @@ namespace bleinput {
 // Advertised central name shown to peripherals during pairing.
 inline constexpr const char* kHostName = "CrossPoint";
 
+// Heap floor for starting the NimBLE stack (~57 KB) while leaving the reader's
+// section-build pre-flight (40 KB) satisfiable afterwards. Shared by the main-loop
+// lifecycle gate and the reader menu's toggle (which offers a defrag restart when a
+// user turns BT on below the floor).
+inline constexpr size_t kStartMinFreeHeap = 100 * 1024;
+
 // Start the BLE HID host (idempotent). Returns false if BLE is compiled out or
 // NimBLE init failed. Safe to call repeatedly.
 bool ensureStarted();