feat(ui): Sync rules management UI (#28e)

[jaybarden1]

Parent

Sub-issue of #28.

Summary

Add sync rules management to the per-account settings panel: list existing include/exclude rules, add new rules, remove rules. Changes take effect on the next sync pass.

Scope (~3 production files)

• SyncRulesViewModel — ReactiveObject; ObservableCollection<SyncRuleItem> bound to persisted SyncRuleEntity rows; AddRuleCommand (path + type), RemoveRuleCommand; persists via ISyncRuleRepository
• SyncRulesView.axaml + .axaml.cs — list of rules with include/exclude badge, add-row form (path input + type toggle), remove button per row
• AccountSettingsViewModel — extend from #28d to expose SyncRulesViewModel

Acceptance criteria

• Existing sync rules are listed with include/exclude indicator
• New rules can be added (path + type); persisted immediately
• Rules can be removed; removal persisted immediately
• Added/removed rules take effect on the next scheduled or manual sync pass
• dotnet build zero warnings; dotnet test all pass

Depends on

• #28d (per-account settings — AccountSettingsViewModel host)