feat(chat): add /detect slash command for mid-session re-detection

[michaelroy-amd]

Summary

• Adds a /detect [accept|save|dismiss] slash command so a local chat
  engine can be re-detected after ChatConsent::Accepted, without a
  dedicated keybinding colliding with focused chat text entry.
• Bare /detect re-runs the same probe used at startup
  (detect_local_chat: managed-services registry, then Lemonade
  :13305 / vLLM :8000 / rocm serve :11435) and echoes the result into
  the chat transcript.
• /detect accept / /detect save / /detect dismiss route through
  the existing accept_detect_offer / save_detect_offer /
  dismiss_detect_offer reducer methods.

Root cause

Once chat is ChatConsent::Accepted, the pre-accept 'd' gate key is
unreachable: while a text field is focused a bare d keypress is
ordinary chat input, and when unfocused the global hotkey table does
not bind 'd' for the Chat tab. There was no way to react to a local
engine coming online (or changing) after the initial handshake short
of restarting the app.

Fix

• slash.rs: new "detect" branch dispatches to the pre-existing
  AppState reducer methods used by the pre-accept offer UI, so no
  new detection or endpoint-swap plumbing was introduced.
• app/mod.rs:
  • request_detect() now also echoes a "Detecting…" message into the
    transcript when already Accepted (previously this messaging only
    made sense pre-accept, driving the gate UI).
  • set_detect_result() echoes the found/not-found result into the
    transcript when Accepted, since draw() only renders the
    offer y/s/n prompt pre-accept (confirmed in ui/tabs/chat.rs) —
    without this the probe result would be silently discarded once
    accepted.
• Accepting or saving a re-detected endpoint via /detect accept /
  /detect save goes through the same accept_detect_offer /
  save_detect_offer calls the initial pre-accept flow uses, so it
  raises the chat_endpoint_rebuild dispatch edge (introduced in
  PR fix(dash): rebuild live chat agent when a local endpoint is accepted #81) exactly like the initial accept does, swapping the live
  agent to the newly detected endpoint.

Coordination notes

• This touches key/slash routing (app/mod.rs, slash.rs), a
  different surface than PR fix(dash): prefer a local chat server over the cloud default at startup #100 (fix/chat-startup-local-first,
  which only touches the event_loop() startup block) and than
  unmerged upstream PR fix(dash): discover served model for configured chat endpoint #97 (which touches resolve_llm_config's
  fallback and the startup block). No overlapping hunks expected.

Relates to EAI-7354.

Test Plan

• cargo build -p rocm-dash-tui
• cargo clippy -p rocm-dash-tui --all-targets --all-features (clean, -D warnings)
• cargo test -p rocm-dash-tui -- --test-threads=1 (551 passed, 0 failed)
• New tests:
  • slash_detect_probes_and_echoes_result_while_accepted
  • slash_detect_accept_raises_endpoint_rebuild_like_initial_accept
  • slash_detect_dismiss_and_unknown_subcommand