fix: effort switch shows by default + status shows the actual model

[SergKam]

Follow-up to #20, fixing two issues reported in testing.

1. No effort switch

The Effort section was keyed on the explicit model override, which is None until the user picks a model — so in the default state (the common case) it never rendered. Now an effective model is resolved (override → the CLI \default` entry → first model) and the effort levels + the model checkmark key off that. The Effort section now appears for the default model too, populated from its \supportedEffortLevels`.

2. Status bar showed "Default", not the real model

The status now surfaces the model the CLI actually resolved, taken from the `Started` event (previously discarded) into a new `active_model` field — e.g. `claude-opus-4-8` (the `[1m]` context suffix is stripped). It updates every turn.

• Before the first turn (or after reopening a tab): falls back to the effective selection's display name (`Default (recommended)`).
• The instant you send a message: resolves to the actual model id.
• Picking a new model clears `active_model` so the new choice shows immediately, then the next turn confirms the resolved id.

Scope

• `state`: `PanelConfig.active_model`; `PanelState.model_status_label`.
• `agent_panel`: `effective_model_value` + `clean_model_id` helpers; `rebuild_model_menu`/`model_effort_status` key off the effective model; `update_model_status` shared with the event handler; menu actions clear `active_model` on model change.
• `event_handler`: `Started` writes `active_model` and refreshes the status.

Checks

fmt ✅ · clippy `-D warnings` ✅ · tests ✅ (105)