docs: record FOSS library evaluation decisions for current custom implementations

Summary

Several custom implementations in this project were written instead of adopting existing FOSS libraries. The rationale is undocumented, making future contributors unaware of the tradeoffs. This issue tracks adding decision records (inline comments or ADRs) for the following:

Files to Document

File	Custom Solution	Known Alternative	Decision to Record
src/contexts/ThemeContext.tsx	Custom dark/light/system theme	next-themes	Sufficient for current needs; revisit if SSR is added
src/utils/searchUtils.ts	Custom Levenshtein search	Fuse.js	Acceptable for small datasets; see #
src/lib/utils.ts (formatToIST)	Custom IST formatter	date-fns-tz	Single use case; see #
src/contexts/AuthContext.tsx	Custom auth + multi-tab sync	—	Justified: interceptor coupling + multi-tab sync too project-specific

Acceptance Criteria

• Each file listed above has a comment referencing the tradeoff
• Or a lightweight ADR exists in docs/decisions/

Labels: documentation, tech-debt, dx