chore(lint): fix golangci-lint issues

- Remove duplicate package comments (godoclint)
- Improve code style (gocritic: assignOp, elseif, emptyStringTest)
- Extract repeated format strings to constants (goconst)
- Fix naming conventions: OriginalUrl -> OriginalURL (revive)
- Wrap external errors with context (wrapcheck)
- Disable gocognit for test files in .golangci.yml

Remaining issues by design:
- funlen: getDefaultCSS (CSS content)
- revive: interfaces package name (meaningful in context)

Author: kjanat

.golangci.yml

@@ -154,6 +154,7 @@ linters:
           - goconst
           - funlen
           - goerr113
+          - gocognit
 
       # Exclude benchmarks from some linters
       - path: _bench_test\.go