refactor(format): centralize currency formatters and app constants

[mrnetwork0001]

Closes #138

Centralizes duplicated currency formatters and scattered magic config values into shared modules, eliminating inline redefinitions across 6 files.

Changes

• Move formatUsdc, formatUsdShortcut to src/utils/format.ts (single source of truth)
• Add formatPrice (3-decimal) helper for per-request pricing
• Create src/config/constants.ts for EXPLORER_BASE_URL, API_BASE_URL, MIN_DEPOSIT, NETWORK_FEE, PRESET_AMOUNTS, LOADING_DELAY_MS
• Remove inline duplicate formatters from App.tsx and ApiUsage.tsx
• Remove per-component currency closures from ApiCard.tsx and ApiDetailPage.tsx
• Replace hard-coded loading delays with LOADING_DELAY_MS constant
• Add Vitest + unit tests for format module (22 tests, 100% coverage)
• Fix pre-existing build errors: duplicate Dashboard import, duplicate statusFilter, missing Skeleton import
• Update README project layout to document new modules

Verification

• npm run build ✅
• npm run test - 22/22 passed ✅
• Rendered output is unchanged across all affected components

Note on LOADING_DELAY_MS

The issue asks for a single shared LOADING_DELAY_MS constant, but also states that simulated loading durations should remain unchanged. These are slightly contradictory since Dashboard previously used 1200ms while MarketplacePage and ApiDetailPage both used 1500ms. I unified all three to 1500ms to keep a single constant as requested - this means the Dashboard skeleton shows for 300ms longer, which is imperceptible to users. Happy to split into per-component constants if preferred.

Closes #138

[greatest0fallt1me]

yep, the formatUsdc copies scattered across files were overdue for this. one source of truth plus the new format tests is exactly what i wanted. merging.