feat: add types.Category, data models, and browserdata.Data (#512)

* 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)

types/models.go

@@ -0,0 +1,71 @@
+package types
+
+import "time"
+
+// LoginEntry represents a single saved login credential.
+type LoginEntry struct {
+	URL       string    `json:"url"        csv:"url"`
+	Username  string    `json:"username"   csv:"username"`
+	Password  string    `json:"password"   csv:"password"`
+	CreatedAt time.Time `json:"created_at" csv:"created_at"`
+}
+
+// CookieEntry represents a single browser cookie.
+type CookieEntry struct {
+	Host       string    `json:"host"        csv:"host"`
+	Path       string    `json:"path"        csv:"path"`
+	Name       string    `json:"name"        csv:"name"`
+	Value      string    `json:"value"       csv:"value"`
+	IsSecure   bool      `json:"is_secure"   csv:"is_secure"`
+	IsHTTPOnly bool      `json:"is_http_only" csv:"is_http_only"`
+	ExpireAt   time.Time `json:"expire_at"   csv:"expire_at"`
+	CreatedAt  time.Time `json:"created_at"  csv:"created_at"`
+}
+
+// BookmarkEntry represents a single browser bookmark.
+type BookmarkEntry struct {
+	Name      string    `json:"name"       csv:"name"`
+	URL       string    `json:"url"        csv:"url"`
+	Folder    string    `json:"folder"     csv:"folder"`
+	CreatedAt time.Time `json:"created_at" csv:"created_at"`
+}
+
+// HistoryEntry represents a single browser history record.
+type HistoryEntry struct {
+	URL        string    `json:"url"         csv:"url"`
+	Title      string    `json:"title"       csv:"title"`
+	VisitCount int       `json:"visit_count" csv:"visit_count"`
+	LastVisit  time.Time `json:"last_visit"  csv:"last_visit"`
+}
+
+// DownloadEntry represents a single browser download record.
+type DownloadEntry struct {
+	URL        string    `json:"url"         csv:"url"`
+	TargetPath string    `json:"target_path" csv:"target_path"`
+	TotalBytes int64     `json:"total_bytes" csv:"total_bytes"`
+	StartTime  time.Time `json:"start_time"  csv:"start_time"`
+	EndTime    time.Time `json:"end_time"    csv:"end_time"`
+}
+
+// CreditCardEntry represents a single saved credit card.
+type CreditCardEntry struct {
+	Name     string `json:"name"      csv:"name"`
+	Number   string `json:"number"    csv:"number"`
+	ExpMonth string `json:"exp_month" csv:"exp_month"`
+	ExpYear  string `json:"exp_year"  csv:"exp_year"`
+}
+
+// StorageEntry represents a single key-value pair from local or session storage.
+type StorageEntry struct {
+	URL   string `json:"url"   csv:"url"`
+	Key   string `json:"key"   csv:"key"`
+	Value string `json:"value" csv:"value"`
+}
+
+// ExtensionEntry represents a single browser extension.
+type ExtensionEntry struct {
+	Name        string `json:"name"        csv:"name"`
+	ID          string `json:"id"          csv:"id"`
+	Description string `json:"description" csv:"description"`
+	Version     string `json:"version"     csv:"version"`
+}