Refactor code structure for improved readability and maintainability

[Nursca]

Summary

Here's a summary of what changed:

Closes #202
src/lib/constants.ts — Imported Category, PolicyStatus, ClaimStatus from @/types and typed the lookup tables:

• CATEGORY_LABELS: Record<string, string> → Record<Category, string>
• CATEGORY_ICONS: Record<string, string> → Record<Category, string>
• STATUS_COLOURS: Record<string, string> → Record<PolicyStatus | ClaimStatus, string>

src/components/Badge.tsx — Tightened types to satisfy the stricter lookup tables:

• Added ColourName type for badge colour values
• variant prop typed as ColourName instead of string
• Added isStatusKey type guard for safe indexing into STATUS_COLOURS

Closes #203
Added security headers to netlify.toml:

• X-Frame-Options: DENY + frame-ancestors 'none' — prevents iframing/clickjacking
• X-Content-Type-Options: nosniff — prevents MIME sniffing
• Referrer-Policy: strict-origin-when-cross-origin — limits referrer leakage
• Permissions-Policy — disables camera, microphone, geolocation
• Content-Security-Policy — restricts origins to self, Stellar Horizon/Soroban RPCs, PostHog, and Google Fonts; blocks all other external connections

Closes #204
The fix adds safeBigInt to src/lib/format.ts:3 which validates via the same /^-?\d+$/ regex as stroopsToDisplay before converting, returning 0n for any malformed input. The dashboard reduce calls at src/app/dashboard/page.tsx:27-28 now use it instead of raw BigInt().

Closes #205
The "Active Policies" section now branches on error before checking active.length === 0 (src/app/dashboard/page.tsx:76). When a fetch error occurs, it shows an inline notice instead of the misleading empty state.

[netlify]

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