feat(ui): improve UX with Pin mode, scrollable alerts, and sparkline enhancements

[TakahashiMinato]

Description

This PR introduces several UI/UX improvements to ClashBar, porting and adapting fixes from the fix/ui branch to the current codebase structure.

1. Pin Mode (Window Stickiness)

Introduces a "Pin" functionality to keep the main application window visible even when clicking outside or switching spaces.

• Toggle Button: Added a pin icon in the MenuBar header with dynamic states (pin / unpin).
• Window Behavior: When pinned, the panel adjusts its collectionBehavior to .canJoinAllSpaces and disables hidesOnDeactivate.
• Event Handling: Prevents the window from closing on external mouse clicks when the pin mode is active.

2. Scrollable NSAlerts

Improves the presentation of long text in system alerts (e.g., configuration validation errors or remote import logs).

• Custom Extension: Added NSAlert+Scrollable to wrap informative text in a dynamically sized NSScrollView.
• UX Fix: Prevents alert windows from growing taller than the screen height, ensuring "OK" buttons remain accessible.
• Applications: Applied to core failure alerts, config validation, and subscription import results.

3. Traffic Sparkline Optimization

Refines the real-time traffic chart for better visual consistency.

• Fixed Window: Enforces a consistent 60-point sliding window (X-axis).
• Visual Alignment: Charts now align to the right, naturally filling up as data accumulates.
• Path Correction: Fixed area filling logic to ensure gradients are correctly bounded regardless of data points count.

4. Localization

• Added ui.action.pin and ui.action.unpin strings for both English and Simplified Chinese.

Changes

• Core: Added NSAlert+Scrollable.swift extension.
• ViewModels: Updated AppViewModel for state management and AppViewModel+Alerts for modal logic.
• Views: Modified MenuBarHeaderView, TrafficSparklineView, and StatusItemController.
• Resources: Updated Localizable.strings for multiple languages.

Verification Results

• Build successful via make build.
• Verified Pin mode stays active across space switches.
• Verified long error messages are scrollable in NSAlerts.
• Verified Traffic Sparkline correctly renders from right-to-left.

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