feat(web): slash command autocomplete + /status /list + fix chat input locking (#404)

* feat(web): slash command autocomplete, /status /list /cancel, fix input locking

Backend:
- Add JobStatus, JobList, JobCancel Submission variants to submission.rs
- Parse /status [id], /progress [id], /list, /cancel <id> as control commands
- Dispatch to existing handle_check_status/handle_list_jobs/handle_cancel_job
  handlers via new process_job_status/process_job_list/process_job_cancel methods
- Add 4 parser tests (34 total, all passing)

Web UI:
- Add slash command autocomplete: type / in chat input to see all 18 commands
  with descriptions; arrow-key navigation, Tab/Enter to select, Escape to close
- Remove chat input locking: drop textarea.disabled + sendBtn.disabled so users
  can always type and send (including /interrupt while agent is processing)
- Remove quick-action toolbar buttons (↩↪⏸⊖🗑📋) added in previous session
- Remove dead #chat-status bar (min-height 28px black bar always visible when empty)

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* refactor: address PR review comments

- Remove Submission::JobList variant; parse /list directly as
  JobStatus { job_id: None } (simpler, eliminates redundant enum
  variant, match arm, is_control branch, and wrapper function)
- Cache autocomplete matches in _slashMatches to avoid re-filtering
  SLASH_COMMANDS on every keydown while autocomplete is open

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>
Co-authored-by: Pierre LE GUEN <[email protected]>

src/agent/agent_loop.rs

@@ -717,6 +717,13 @@ impl Agent {
             Submission::Heartbeat => self.process_heartbeat().await,
             Submission::Summarize => self.process_summarize(session, thread_id).await,
             Submission::Suggest => self.process_suggest(session, thread_id).await,
+            Submission::JobStatus { job_id } => {
+                self.process_job_status(&message.user_id, job_id.as_deref())
+                    .await
+            }
+            Submission::JobCancel { job_id } => {
+                self.process_job_cancel(&message.user_id, &job_id).await
+            }
             Submission::Quit => return Ok(None),
             Submission::SwitchThread { thread_id: target } => {
                 self.process_switch_thread(message, target).await