Add assistant cost usage limits to Settings

[nightnei]

Related issues

• Fixes STU-1820

How AI was used in this PR

AI helped with writing code, iterated, I reviewed carefully and adjusted.

Proposed Changes

We will display percentage of usage limits based on cost in Studio Settings.

Testing Instructions

1. Apply these wpcom (223470-ghe-Automattic/wpcom) changes
2. Set 'cost_usage' => 1500000, in class-studio-app-ai-assistant.php
3. Open Studio Settings and assert that you see the correct 7.5%:
   
4. Set 'cost_usage' => 30000000, (bigger than the cap)
5. Assert that you see 100% in Studio Settings

[wpmobilebot]

📊 Performance Test Results

Comparing a57b3a2 vs trunk

app-size

Metric	trunk	a57b3a2	Diff	Change
App Size (Mac)	2328.95 MB	2334.13 MB	+5.18 MB	🔴 0.2%

site-editor

Metric	trunk	a57b3a2	Diff	Change
load	1761 ms	1744 ms	17 ms	⚪ 0.0%

site-startup

Metric	trunk	a57b3a2	Diff	Change
siteCreation	8040 ms	8531 ms	+491 ms	🔴 6.1%
siteStartup	3876 ms	3882 ms	+6 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)