feat: add cost dashboard (Usage tab) to Settings

[SpaceHunterInf]

• New UsageTab component showing total spend, daily average, API calls, and token usage with an interactive bar chart
• Time range selector (7d / 14d / 30d) for adjustable cost windows
• Token breakdown visualization (input vs output) with proportional bars
• Consumes existing backend routes: /config/cost/global, /config/cost/daily
• Added fetchGlobalCost and fetchDailyCost API client functions
• Full i18n support (English + Simplified Chinese)
• Dollar-sign icon in settings sidebar between Data and Appearance tabs

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• Test A
• Test B

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes