Add Target HF leverage solver

[Sikkra]

Closes #5

Summary

• Adds a Target HF numeric input beside the leverage controls.
• Solves leverage from the target health factor and updates slider, leverage input, supplied/borrowed totals, and HF preview together.
• Rejects target HF values below 1 / c_factor with an inline error and preserves the previous slider value.
• Keeps slider-driven updates bidirectional by syncing the Target HF field from the current leverage/HF preview.

Verification

• npm run build (passes; existing Vite chunk-size warning remains)
• git diff --check
• npx tsc --noEmit (still fails on existing repo type/config issues: .ts import extension settings, AuthModalParams.network, and the main-branch demo reserve mock missing ReserveStats fields)
• Local Edge/Playwright fallback on http://127.0.0.1:5204/: HF 1.8 solved leverage to 1.8x, preview HF rendered 1.8000, supplied/borrowed rendered 1,800.00 / 800.00; invalid HF 1.2 showed the 1.2500 floor error without changing slider; slider 2.0x updated Target HF and preview to 1.6000; browser errors 0.

Note: the Codex in-app browser pane was unavailable in this session, so I used the local Edge/Playwright fallback for UI verification.