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/repository/src/commonMain/kotlin/org/meshtastic/core/repository/RemoteShellHandler.kt

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2025-2026 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.meshtastic.core.repository
+
+import kotlinx.coroutines.flow.SharedFlow
+import org.meshtastic.proto.MeshPacket
+import org.meshtastic.proto.RemoteShell
+
+/**
+ * A decoded [RemoteShell] frame together with the node number that sent it.
+ *
+ * Propagating [from] allows downstream consumers (e.g. the ViewModel) to verify that a frame
+ * actually originated from the expected peer rather than relying solely on [RemoteShell.session_id].
+ */
+data class ReceivedShellFrame(
+    val from: Int,
+    val frame: RemoteShell,
+)
+
+/**
+ * Interface for handling RemoteShell packets (REMOTE_SHELL_APP portnum = 13).
+ *
+ * RemoteShell is a PTY-over-mesh feature that relays a shell session across the mesh network. The firmware-side
+ * implementation is currently unreleased (gated to [Capabilities.supportsRemoteShell]).
+ */
+interface RemoteShellHandler {
+    /**
+     * The most recently received [ReceivedShellFrame], emitted to collectors.
+     *
+     * Uses [SharedFlow] (not `StateFlow`) so that rapid or identical frames are never silently dropped.
+     */
+    val lastFrame: SharedFlow<ReceivedShellFrame>
+
+    /**
+     * Processes an incoming RemoteShell packet.
+     *
+     * @param packet The received mesh packet carrying a [RemoteShell] payload.
+     */
+    fun handleRemoteShell(packet: MeshPacket)
+}