feat: API key management improvements

- Rename routes: /dashboard/tokens → /dashboard/apikeys, /dashboard/manager/keys → /dashboard/manager/channels
- Add KeyPlain field to store plaintext API keys for re-viewing
- API key list shows masked key (sk-ot-123456****abcd) with eye toggle to reveal
- Copy button with 2s feedback on key list
- TokenNew shows full key + copy after creation
- Increase key prefix display to 12 characters
- Fix SQLite driver: replace gorm.io/driver/sqlite with ncruces/go-sqlite3/gormlite
- Fix user.status === 'active' checks across frontend views
- Add channel store for new channels API
- Update Makefile frontend build to work reliably

BREAKING CHANGE: Existing API keys created before this change will not show their plaintext value (only prefix visible).

backend/internal/store/models.go

@@ -58,6 +58,7 @@ type APIKey struct {
 	UserID              uint64     `gorm:"index;not null" json:"user_id"`
 	Name                string     `gorm:"size:64;not null" json:"name"`
 	KeyHash             string     `gorm:"uniqueIndex;size:64;not null" json:"-"`
+	KeyPlain            string     `gorm:"size:255;not null" json:"key_plain"`
 	KeyPrefix           string     `gorm:"size:32;not null" json:"key_prefix"`
 	QuotaTokensPerDay   *int64     `json:"quota_tokens_per_day,omitempty"`
 	QuotaRequestsPerDay *int       `json:"quota_requests_per_day,omitempty"`