Polish settings UI and restore development app icon

[IchenDEV]

Summary

• make the settings window fixed-size and restyle General, Models, Style, Integrations, and About with native grouped forms
• integrate the Activity mode switch into page controls and align history content to the shared grid
• put Qwen first with a recommendation marker and make Custom an independent formatting-model type
• restore Dock and LaunchServices icon metadata and root icon assets in development app bundles

Validation

• 565 XCTest tests passed, with 8 intentional integration skips; the Swift Testing policy test passed
• basic CI checks and SDLC validation against origin/main passed
• development build, strict code-signature verification, app launch, and icon package invariant passed
• real-window QA covered Chinese and English, light and dark appearances, all settings tabs, both Activity modes, and the Custom model state

Review boundary

• AppSettings.swift is classified as a high-risk control surface by repository policy, so independent verification and normal human approval remain required before merge
• no signing identity, release tag, production permission, model ID, or persisted enum raw value changed