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]>

Cargo.lock

@@ -955,6 +955,18 @@ dependencies = [
  "crossbeam-utils",
 ]
 
+[[package]]
+name = "console"
+version = "0.15.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "054ccb5b10f9f2cbf51eb355ca1d05c2d279ce1804688d0db74b4733a5aeafd8"
+dependencies = [
+ "encode_unicode",
+ "libc",
+ "once_cell",
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "const-oid"
 version = "0.9.6"
@@ -1389,7 +1401,7 @@ checksum = "f46882e17999c6cc590af592290432be3bce0428cb0d5f8b6715e4dc7b383eb3"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.116",
+ "syn 2.0.117",
 ]
 
 [[package]]