feat: queue prompts while the agent is running (#598)

Allow the chat input to be sent while the agent is mid-turn or awaiting
approval. The backend's prompt.submit busy-input policy already queues any
prompt that lands during a running turn (tui_gateway/server.py:_handle_busy_submit)
and drains it as the next turn, so the message is never dropped.

- ChatInputPolicy: pure, testable send/placeholder decision logic
- ChatScreen: canSend stays enabled while typing/approving; placeholder shows
  'Queued — sends when agent is free' when text is present during a busy turn
- ChatInputPolicyTest: truth-table covering typing, approval-pending,
  disconnected, empty, attachment, and slash-command cases
- strings: chat_input_placeholder_queue

Note: this supersedes the Genesi599 fork's approach (dispatchViaRpc('queue ...')),
which double-submits and relies on a 4018-prone command.dispatch path. The
backend-queued prompt.submit path is the correct, single-submission contract.

Co-authored-by: M57 <m57@hermes-control.app>

app/src/main/res/values/strings.xml

@@ -112,6 +112,7 @@
     <string name="chat_reasoning_collapse">Collapse reasoning</string>
     <string name="chat_input_placeholder_not_connected">Not connected\u2026</string>
     <string name="chat_input_placeholder_waiting">Waiting for response\u2026</string>
+    <string name="chat_input_placeholder_queue">Queued \u2014 sends when agent is free\u2026</string>
     <string name="chat_input_placeholder_type_message">Type a message\u2026</string>
     <string name="chat_tool_stderr">Error Output (stderr):\n%1$s</string>
     <string name="chat_tool_execution_error">Execution Error: %1$s</string>