feat(store): persist garmin_connected_at, set once on first successful auth

docs/DATABASE.md

@@ -35,6 +35,12 @@ CREATE TABLE profile (
     name                TEXT NOT NULL DEFAULT 'Default',
     garmin_email        TEXT NOT NULL DEFAULT '',
     garmin_password     TEXT NOT NULL DEFAULT '',
+    -- Set once, the first time this user successfully authenticates with
+    -- Garmin (handleAuthLogin/handleAuthMFA). Null means never connected --
+    -- the login gate uses this (not the in-memory auth status, which
+    -- resets on restart) to decide whether a returning user must
+    -- reconnect before entering the app.
+    garmin_connected_at TEXT,
     rolling_window_days INTEGER NOT NULL DEFAULT 90,
     -- BackfillHorizonDays bounds how far back "Sync now" reaches when
     -- walking backward from today; read fresh on every sync (not cached at