Fix: Resolve Go and Flutter compilation errors

Go backend:
- Fix WOPI models file (remove duplicate package declaration and syntax errors)
- Add GetOrgMember method to database as alias for GetUserMembership
- Add UpdateFileSize method to database
- Remove unused net/url import from wopi_handlers.go
- Fix field names in WOPI struct initializers (match JSON tags)

Flutter frontend:
- Remove webview_flutter import (use simpler placeholder for now)
- Fix _createWOPISession to safely access SessionBloc state
- Replace WebViewController usage with placeholder UI
- Remove unused _generateRandomHex methods from login/signup forms
- Add missing mimeType parameter to DocumentCapabilities in mock repository
- Remove unused local variables in file_browser_bloc

go_cloud/internal/database/db.go

@@ -224,6 +224,11 @@ func (db *DB) GetUserMembership(ctx context.Context, userID, orgID uuid.UUID) (*
 	return &membership, nil
 }
 
+// GetOrgMember is an alias for GetUserMembership - checks if user is a member of an org
+func (db *DB) GetOrgMember(ctx context.Context, orgID, userID uuid.UUID) (*Membership, error) {
+	return db.GetUserMembership(ctx, userID, orgID)
+}
+
 func (db *DB) CreateOrg(ctx context.Context, ownerID uuid.UUID, name, slug string) (*Organization, error) {
 	var org Organization
 	err := db.QueryRowContext(ctx, `
@@ -483,6 +488,16 @@ func (db *DB) GetFileByID(ctx context.Context, fileID uuid.UUID) (*File, error)
 	return &f, nil
 }
 
+// UpdateFileSize updates the size and modification time of a file
+func (db *DB) UpdateFileSize(ctx context.Context, fileID uuid.UUID, size int64) error {
+	_, err := db.ExecContext(ctx, `
+		UPDATE files
+		SET size = $1, last_modified = NOW()
+		WHERE id = $2
+	`, size, fileID)
+	return err
+}
+
 // DeleteFileByPath removes a file or folder matching path for a given org or user
 func (db *DB) DeleteFileByPath(ctx context.Context, orgID *uuid.UUID, userID *uuid.UUID, path string) error {
 	var res sql.Result