feat(remote-shell): PTY-over-mesh terminal with retro-CRT UI

Adds a full RemoteShell (portnum=13) implementation matching Jonathan's
dmshell_client.py protocol:

Protocol layer:
- Seq/ack reliability: incrementing seq on every non-ACK frame, piggybacked
  ack_seq, out-of-order frame buffering, gap detection and replay requests
- TX history ring buffer (last 50 frames) for retransmission on request
- ACK frames carry optional 4-byte big-endian REPLAY_REQUEST payload
- PING/PONG heartbeat with 8-byte status payload (lastTxSeq, lastRxSeq);
  PONG handler triggers replay if peer is behind
- PKI: DataPacket.PKC_CHANNEL_INDEX so CommandSenderImpl applies
  Curve25519 encryption (firmware rejects non-PKI DMShell packets)
- Input batching: 500ms debounce (matches Python client), immediate flush
  on \r, \t, buffer-full (64 bytes), or Enter

Terminal UI:
- Retro-CRT composables: TerminalCanvas (phosphor glow, two-pass bloom),
  ScanlinesOverlay, FlickerEffect (animated brightness variation),
  CrtCurvatureModifier (AGSL barrel distortion on Android 12+, no-op on JVM)
- PhosphorPreset enum: GREEN (P1), AMBER (P3), WHITE (P4)
- Pending-input rendered inline in preset.dim colour; snaps to confirmed on flush
- Hidden zero-size BasicTextField captures soft and hardware keyboard input
- Phosphor colour picker dropdown in top bar

Capabilities gate:
- supportsRemoteShell gated to UNRELEASED (9.9.9)
- Entry only visible in AdministrationSection when node.capabilities.supportsRemoteShell

core/model/src/commonMain/kotlin/org/meshtastic/core/model/Capabilities.kt

@@ -64,6 +64,12 @@ data class Capabilities(val firmwareVersion: String?, internal val forceEnableAl
     /** Support for ESP32 Unified OTA. Supported since firmware v2.7.18. */
     val supportsEsp32Ota = atLeast(V2_7_18)
 
+    /**
+     * Support for the RemoteShell module (PTY-over-mesh, REMOTE_SHELL_APP portnum). Defined in protobufs HEAD
+     * (post-v2.7.21); gated to [UNRELEASED] until a firmware release ships it.
+     */
+    val supportsRemoteShell = atLeast(UNRELEASED)
+
     companion object {
         private val V2_6_8 = DeviceVersion("2.6.8")
         private val V2_6_9 = DeviceVersion("2.6.9")

core/model/src/commonTest/kotlin/org/meshtastic/core/model/CapabilitiesTest.kt

@@ -85,6 +85,12 @@ class CapabilitiesTest {
         assertTrue(caps("2.7.19").supportsTakConfig)
     }
 
+    @Test
+    fun supportsRemoteShell_is_currently_unreleased() {
+        assertFalse(caps("2.7.22").supportsRemoteShell)
+        assertFalse(caps("3.0.0").supportsRemoteShell)
+    }
+
     @Test
     fun supportsEsp32Ota_requires_V2_7_18() {
         assertFalse(caps("2.7.17").supportsEsp32Ota)
@@ -104,6 +110,7 @@ class CapabilitiesTest {
         assertFalse(c.supportsStatusMessage)
         assertFalse(c.supportsTrafficManagementConfig)
         assertFalse(c.supportsTakConfig)
+        assertFalse(c.supportsRemoteShell)
         assertFalse(c.supportsEsp32Ota)
     }
 
@@ -115,5 +122,6 @@ class CapabilitiesTest {
         assertTrue(c.supportsStatusMessage)
         assertTrue(c.supportsTrafficManagementConfig)
         assertTrue(c.supportsTakConfig)
+        assertTrue(c.supportsRemoteShell)
     }
 }