DRY refactor, plan form extraction & Binance USDC (v2.6.1)

[nehasvit]

Summary

• CredentialFormDelegate — shared credential form state extracted from 4 ViewModels (AddPlan, Onboarding, AddExchange, ExchangeDetail), eliminating ~11 duplicate fields and 6 methods per ViewModel
• PlanFormDelegate + PlanFormContent — shared plan form logic and UI extracted from AddPlan, Onboarding FirstPlan, and EditPlan screens
• ApiImportResultDialog — shared composable replacing ~30-line inline AlertDialog blocks in 3 screens
• AccBotTopAppBar migration — 11 screens migrated from raw TopAppBar to the shared component
• KuCoin signedRequest() helper extracted, NumberFormatters.roiValues() added
• Binance: USDT → USDC, min order size lowered to 5 EUR/USDC
• Quick amount presets: 5, 10, 25, 50, 100 (was 25–500)
• Default DCA plan amount set to exchange minimum on exchange selection
• Min order size display strips trailing zeros
• USDC icon and MarketDataService mapping added
• Net: -321 lines (1923 insertions, 2244 deletions) across 45 files

Test plan

• Create a new DCA plan on Binance — verify USDC appears (not USDT), default amount is 5
• Create a new DCA plan on other exchanges — verify correct fiat/min defaults
• Quick amount buttons show 5, 10, 25, 50, 100
• Min order size label has no trailing zeros (e.g. "5" not "5.0000000")
• Onboarding flow: exchange setup → credential validation → first plan creation
• Add/edit exchange credentials on all supported exchanges
• API import result dialog appears correctly on AddExchange, ExchangeDetail, PlanDetails
• Top app bar with back navigation works on all screens
• ROI calculation on Dashboard and PlanDetails matches expected values
• What's New dialog shows v2.6.1 release notes

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