docs: add CODESTYLE.md with Clean Code, DRY/KISS, and SOLID principles

Covers naming, function design, guard clauses, comment policy, command-query
separation, DRY vs KISS trade-offs (KISS wins), and SOLID applied to the
Java backend and TypeScript/Svelte frontend. Linked from CLAUDE.md and
COLLABORATING.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

COLLABORATING.md

@@ -140,9 +140,12 @@ fix(e2e): clear locale cookie when switching back to base language
 
 When in doubt, commit more often rather than less.
 
-## Code Style Reminders
+## Code Style
 
+See [CODESTYLE.md](./CODESTYLE.md) for the full guide: Clean Code (Uncle Bob), DRY/KISS trade-offs, and SOLID principles applied to this stack.
+
+Quick reminders:
 - Pure functions over stateful helpers where possible
-- No premature abstractions — solve the problem in front of you
+- No premature abstractions — KISS beats DRY
 - No backwards-compatibility shims for code that has no callers
 - Validate at system boundaries only (user input, external APIs)