feat(ui): add user setting to control target="_blank" on links

Rationale: Opening links in the current tab is the default browser behavior.

Using `target="_blank"` on external links can lead to accessibility issues and override user preferences. It may also interfere with assistive technologies and expected browser behavior.

To maintain backward compatibility, this option is enabled by default (`true`), which adds `target="_blank"` to links.

internal/database/migrations.go

@@ -1086,4 +1086,8 @@ var migrations = []func(tx *sql.Tx, driver string) error{
 		_, err = tx.Exec(sql)
 		return
 	},
+	func(tx *sql.Tx, _ string) (err error) {
+		_, err = tx.Exec(`ALTER TABLE users ADD COLUMN open_external_links_in_new_tab bool default 't'`)
+		return err
+	},
 }