* feat: add new types.Category, data models, and browserdata.Data
Phase 1 of architecture refactoring (RFC-001/RFC-002):
- types/category.go: Category enum (9 values) replacing DataType (22 values)
with String(), IsSensitive(), AllCategories, NonSensitiveCategories()
- types/models.go: browser-agnostic data models (LoginEntry, CookieEntry,
BookmarkEntry, HistoryEntry, DownloadEntry, CreditCardEntry, StorageEntry,
ExtensionEntry) — no encrypted fields, no browser prefixes
- types/category_test.go: tests for Category methods
- browserdata/browser_data.go: new Data struct with typed slices,
coexists with old BrowserData during migration
* docs: replace Coveralls badge with Codecov in README
* fix: apply review suggestions (is_http_only tag, json tags on Data)
Roger