feat(calendar): add pst_last_path/pst_auto_sync to Settings

Backing fields for remembering the imported .pst path across sessions
and an opt-in startup auto-resync (T6.2). #[serde(default)] so an
existing settings.json without these keys still deserializes.

src-tauri/src/models.rs

@@ -207,6 +207,12 @@ pub struct Settings {
     // Storage retention policy (FR-STORE-2). None = no cap on that dimension.
     pub retention_max_age_days: Option<u32>,
     pub retention_max_size_gb: Option<u32>,
+    // Calendar / .pst (T6.2) — remembered so the user doesn't re-browse every
+    // launch. `pst_auto_sync` re-imports this path once at startup if set.
+    #[serde(default)]
+    pub pst_last_path: Option<String>,
+    #[serde(default)]
+    pub pst_auto_sync: bool,
 }
 
 // ---- Sync (ADR-0010) ----