## Summary
Replaces fragile `strings.Contains(err.Error(), "UNIQUE")` checks with typed error detection using `errors.As` and the SQLite driver's `*sqlite.Error` type.
## Changes
- **`internal/db/errors.go`** (new): Adds `IsUniqueConstraintError(err)` helper that uses `errors.As` to unwrap the error into `*sqlite.Error` and checks for `SQLITE_CONSTRAINT_UNIQUE` (code 2067).
- **`internal/handlers/api.go`**: Replaces two `strings.Contains(err.Error(), "UNIQUE")` calls with `db.IsUniqueConstraintError(err)` — in `handleCreateSessionError` and `executeNickChange`.
- **`internal/handlers/auth.go`**: Replaces one `strings.Contains(err.Error(), "UNIQUE")` call with `db.IsUniqueConstraintError(err)` — in `handleRegisterError`.
## Why
String matching on error messages is fragile — if the SQLite driver changes its error message format, the detection silently breaks. Using `errors.As` with the driver's typed error and checking the specific SQLite error code is robust, idiomatic Go, and immune to message format changes.
closes #39
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Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #66
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