feat: add used and remaining usage display modes

[LiarCoder]

I'm a totally rookie in Rust, so this feature is implemented by Codex. If someone finds this PR is not enough to be merged, welcome to refine it or just decline it, thanks~ 😁

Preview

Overview

• Add a top-level Usage Display menu below Models for switching between Used and Remaining.
• Keep used usage as the default and persist the user's selection via the usage_display setting.
• Apply the selected percentage consistently to the taskbar component text, progress bar, tray badge, and tray tooltip.
• Continue calculating tray warning colors based on used usage so that low remaining usage still triggers a high-usage warning.
• Preserve the placeholder display behavior for loading, authentication failure, and unavailable quota states.
• Add menu translations for all 10 existing languages and update the README instructions.

Verification

• cargo test: all 10 tests passed.
• cargo fmt -- --check: passed.

Related Issue

Closes #40

[LiarCoder]

By the way, should I put the "Usage Display" option into the "Settings" section as its sub-menu?