iOS: per-setting sync table with sync icons, dropdowns, colour resets

Generalise the two-tier SettingsSync into a server-driven flag map: each
syncable key has an account-wide flag (fetched from the server); pull applies
only on-flagged keys, push (read-modify-write) sends only on-flagged keys plus
the flag map. Per-row link icon toggles a setting's sync; a global switch flips
all. Enabling a flag adopts this device's current value as the shared one.

- AppSettings: fix bg_color mapping (was background_color), add cache_months,
  month_view_paged, sync_flags
- SettingsView: uniform rows (sync icon | name | value), ContrastSelector and
  the button pickers replaced by menu dropdowns, every colour gets a reset to a
  canonical default, global "sync all" toggle; account settings (privacy,
  group calendar, hide profile) kept in their own block; language and text/line
  contrast kept device-local (iOS-specific)
- unify default line colour to #3A3A52 across views

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Calendarr iOS/Models/CalendarStore.swift

@@ -579,7 +579,7 @@ class CalendarStore {
             todayColorHex:      defaults.string(forKey: "todayColor")       ?? "#4285f4",
             textColorHex:       defaults.string(forKey: "textColor")        ?? "#FFFFFF",
             backgroundColorHex: defaults.string(forKey: "backgroundColor")  ?? "#000000",
-            lineColorHex:       defaults.string(forKey: "lineColor")        ?? "#3A3A3C",
+            lineColorHex:       defaults.string(forKey: "lineColor")        ?? "#3A3A52",
             primaryColorHex:    defaults.string(forKey: "primaryColor")     ?? "#4285f4",
             accentColorHex:     defaults.string(forKey: "accentColor")      ?? "#ea4335",
             language:           defaults.string(forKey: "appLanguage")      ?? "system"