feat: session mode/model controls in composer (SessionSettingsBar)

Extracted from #39. Gated on decision #62.

Normalize ACP modes/models/configOptions from `session/new` and `session/load` into a webview-safe settings state; add SESSION_SETTINGS / SET_SESSION_MODE / SET_SESSION_MODEL messages; write changes via `session/set_mode`, `session/set_model`, or `session/set_config_option`; render SessionSettingsBar pills with a searchable model menu in the composer, disabled while generating. Gracefully no-ops when the agent exposes no modes/models.

Fix while porting: in #39, agent-pushed `current_mode_update`/`config_option_update` notifications are only consumed during session/load replay, not mid-conversation.

**Size**: M-L | **Depends on**: decision #62; #60 (or hand-rolled types) | **Tier**: 2