feat(acp): add in-session mode/model/agent controls with ACP preference persistence

[iguit0]

Summary

This PR makes ACP mode/model/agent configurable at runtime from the session header and persists those selections per session, with workspace defaults fallback for new/empty sessions. #40

Why

ACP mode/model/agent were not fully controllable from the session UI, and selections were not persisted in a way that supports consistent resume/new-session behavior.

What Changed

• Added header controls for:
• Mode
• Model
• Agent
• Advanced config options
• Added disabled behavior when streaming or disconnected.
• Kept Model/Agent visible and disabled with “Unavailable from ACP” if ACP does not expose those options.
• Added ACP config-option support in Rust and TS types.
• Added new Tauri command:
• acp_set_config_option(workspace_id, session_id, config_id, option_id) mapped to session/set_config_option (option_id -> ACP value).
• Added session/workspace preference persistence:
• sessions.acp_preferences_json column
• workspace_acp_defaults table
• session_update_acp_preferences
• workspace_acp_defaults_get
• workspace_acp_defaults_set
• Updated forget_workspace_data to delete workspace ACP defaults for directory workspaces.
• Implemented preference application order on session init:
• ACP session info
• session preferences
• workspace defaults fallback when session prefs are empty
• apply mode first, then config options
• ignore stale/invalid IDs safely
• Preserved planning workflow auto-switch behavior:
• planning => force plan
• approve => force agent
• append notice message when auto-switch occurs
• workflow-forced mode changes do not overwrite user defaults

Data Contract

acp_preferences_json:

{
  "modeId": "string | null",
  "selectedConfigOptions": {
    "configId": "optionId"
  }
}