fix: OpenRouter frontend + MiniMax zero-quota filter

[prakersh]

Summary

• Adds full frontend support for the OpenRouter provider (PR feat: add OpenRouter provider for credit/usage monitoring #44 only added backend)
• Filters out MiniMax models with zero quotas (speech-hd, Hailuo, music, image-01)

OpenRouter Frontend

• Credits quota card with dollar amount display ($0.0000 / $0.2000)
• Provider detection in getCurrentProvider() (was falling back to Synthetic)
• Chart rendering with Usage, Daily Usage, and Usage % series
• Cycle Overview with "Credits" period category
• Session table with "Credits Used" / "Session Delta" / "Snapshots" headers
• Menubar provider card showing Credits quota
• Settings override dropdown entry
• README documentation

MiniMax Zero-Quota Filter

• Filters total=0 && used=0 models from current view, history, logging history, and tracker
• Reduces card count from 6-7 to just the active models (e.g. MiniMax-M*)
• Applied consistently across all endpoints including menubar

Test Plan

• ./app.sh --test - all 14 packages pass with race detection
• OpenRouter tab renders Credits card, insights, charts, cycle overview
• MiniMax tab shows only active quota models
• Menubar shows OpenRouter with Credits at correct percentage
• No JS console errors
• Anthropic/Codex/other tabs unaffected (regression check)

[codecov]

Codecov Report

❌ Patch coverage is 4.76190% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/web/handlers.go	0.00%	10 Missing and 2 partials ⚠️
internal/web/menubar.go	14.28%	5 Missing and 1 partial ⚠️
internal/tracker/minimax_tracker.go	0.00%	1 Missing and 1 partial ⚠️

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