feat(tui): steer messages into the ongoing stream, add a /settings send-mode switch

[Sayt-0]

Closes #3547.

While the agent is working, a plain send in the full TUI now attaches the message to the ongoing stream via the runtime steer queue (the behavior the lean TUI already has). The previous end-of-turn behavior remains available as a persisted setting: the /custom command becomes /settings, a tabbed dialog whose Behavior tab switches busy sends between Steer (default) and Queue.

Issue expectations

Issue expectation	Implementation
"Send a new message should attach to the ongoing stream"	Plain Enter while working resolves the input and enqueues it on Runtime.Steer; the loop drains it mid-turn and emits UserMessageEvent, so the transcript shows the message where the agent saw it
"A new shortcut like ctrl + q to add messages to queue"	Implemented as a saved setting instead of a shortcut: the Behavior tab of /settings holds a Steer/Queue radio switch, persisted as settings.busy_send_mode in the user config and applied to every tab

The /settings dialog

/custom (layout dialog) is renamed to /settings and gains tabs:

Tab	Content
Visuals	Previous /custom content: live schematic preview, sidebar position, section spacing, section visibility
Behavior	While agent is working: Steer (attach mid-turn, default) or Queue (hold until the turn ends)

Actual render of the Behavior tab:

╭──────────────────────────────────────────────────────────╮
│                                                          │
│                         Settings                         │
│  ──────────────────────────────────────────────────────  │
│                                                          │
│                   Visuals    Behavior                    │
│                                                          │
│  While agent is working                                  │
│                                                          │
│  › ● Steer   send to the working agent mid-turn          │
│    ○ Queue   hold until the current turn ends            │
│                                                          │
│  ↑/↓ navigate  ←/→ change  tab switch tab  enter apply   │
│                        esc cancel                        │
│                                                          │
╰──────────────────────────────────────────────────────────╯

• Enter persists everything (Settings updated toast); Esc cancels and rolls back the layout preview. The send mode never previews, so it needs no rollback.
• With --sidebar=false the dialog opens with only the Behavior tab (previously /custom was rejected entirely). Lean mode still drops the open message: it has no overlay support, an open dialog would invisibly capture keystrokes.
• Defaults are not written out: busy_send_mode is cleared when set back to steer, keeping the config file minimal.

Flow

Enter while busy
  └─ chatPage.handleSendMsg
       ├─ Queue flag / queue send mode / fork-mode skill / no app → local queue
       └─ otherwise → steerMessage (tea.Cmd goroutine)
            ├─ ResolveInput (skill/agent command expansion off the UI loop)
            ├─ App.SteerMessage → Runtime.Steer (attachments become MultiContent)
            ├─ ok → toast; bubble appears when the runtime drains the message
            └─ rejected → steerFailedMsg → re-dispatched with Queue=true (never dropped)

Notes

• Fork-mode skills always queue: they spawn their own stream, which cannot attach to the running one.
• No optimistic transcript bubble: the user message renders when the runtime drains it and emits UserMessageEvent, so transcript order matches what the model actually saw.
• No ctrl+q binding and no new editor_queue action: the key stays free for user remaps and the docs keep their previous examples. SendMsg.Queue remains as an internal flag for the steer-rejection fallback.
• App.Run's attachment processing was extracted to buildUserMultiContent and reused by App.SteerMessage; no behavior change.
• /custom shipped in v1.102.0; no alias is kept for it. If a transition alias seems safer, it can be added.

Testing

Layer	Coverage
Unit	steer on busy send, queue send mode routing (incl. mid-session switch), steer rejection fallback, fork-skill queueing, nil-app fallback, dialog tab switching, radio toggling, apply/cancel semantics, config round-trip (defaults cleared)
E2E	TestChat_SteerWhileStreaming and TestChat_QueueSendModeWhileStreaming drive the real TUI through tuitest against a VCR proxy in simulated-stream mode (250ms per SSE chunk); the queue test flips the switch through the actual /settings dialog and asserts busy_send_mode: queue lands in the config file
Existing suites	pkg/tui/..., pkg/app, pkg/runtime, pkg/userconfig, full e2e/tui; golangci-lint reports 0 issues