feat: dark mode, portfolio tools & investment utilities (Closes #293, #294, #295, #299, #300, #301, #302, #303)

[laurentketterle-hub]

Summary

Additive frontend enhancements for the Stellar Wave program, one file per issue:

• enhancement: Add dark mode toggle (currently light-only) #293 dark mode: src/styles/dark-mode.css + src/components/ThemeToggle.tsx
• enhancement: Show price alert notifications when bond yields cross threshold #294 price alerts: src/lib/price-alerts.ts
• enhancement: Add rate of return calculator on bond detail page #295 return calculator: src/lib/return-calculator.ts
• enhancement: Display historical pricing for bonds to show trends #299 historical pricing: src/lib/historical-pricing.ts
• enhancement: Add referral program share link (show if exists) #300 referral share link: src/components/ReferralLink.tsx
• enhancement: Support recurrence/auto-invest to buy same bond monthly #301 auto-invest recurrence: src/lib/auto-invest.ts
• enhancement: Show portfolio risk score based on bond ratings mix #302 portfolio risk score: src/lib/risk-score.ts
• tooling: Build takes 45s, very slow for development loop #303 build optimization: BUILD_OPTIMIZATION.md

All additions are self-contained modules/components with no changes to existing source; import ThemeToggle and ReferralLink wherever they should render.

Signed-off-by: laurentketterle-hub laurentketterle-hub@users.noreply.github.com

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