Refactoring: src/user/settings.js Function with many returns (count = 14), Function with high complexity (count = 23)

[RogerJinIS]

Summary
This PR refactors [settings.js] to reduce [exports] function complexity and number of returns from cyclomatic analysis reports.

Original issue: Function with many returns (count = 14), Function with high complexity (count = 23) in [exports].
New structure splits logic to standalone internal helpers.
[module.exports] now only maps [User methods to those helpers (single linear assignment path).

What changed

1. Extracted helper functions
   parseJSONSetting(value, defaultValue)
   getSetting(settings, key, defaultValue)
   normalizePageSetting(value, defaultValue, maxValue)
   validatePagination(value, maxValue)
   onSettingsLoaded(uid, settings)
   getSettings(uid)
   getMultipleUserSettings(uids)
   saveSettings(uid, data)
   updateDigestSetting(uid, dailyDigestFreq)
   setSetting(uid, key, value)
2. Simplified module.exports
   Before:
   module.exports = function(User) { ... deep logic, conditionals, multiple returns ... }
   After:
   module.exports = function(User) {
   User.getSettings = getSettings;
   User.getMultipleUserSettings = getMultipleUserSettings;
   User.saveSettings = saveSettings;
   User.updateDigestSetting = updateDigestSetting;
   User.setSetting = setSetting;
   }

Behavior preserved
All existing public functions remain:
.getSettings
.getMultipleUserSettings
.saveSettings
.updateDigestSetting
.setSetting
Input validation and sanitization kept.
Notification type handling preserved.
activitypub and default / remote fallback logic intact.

Notes
This PR is a refactor-only change, no behavior contract changes.
Keep existing integration tests around to validate real-world user settings flow.
Next improvement: add a minor test for getSetting remote fallback plus normalizePageSetting clamps.