Replace the HTML preview with a worker-rendered PDF preview

Rendering moves into a module Web Worker behind a latest-wins RenderClient
with a watchdog and a handshake-based main-thread fallback. The preview
shows the real PDF through pdf.js at 2x with zoom, page indicator, audit
issues popover and a focus modal. A4/Letter and margin controls are frozen
into each invoice at issue and can be saved as the default (migration M3).
react-pdf and pdf.js are lazy-loaded; the main chunk shrinks from 1.8 MB
to 0.5 MB. The HTML twin and its styles are removed.

src-tauri/src/db.rs

@@ -10,7 +10,7 @@ const MAX_BACKUPS: usize = 10;
 const BACKUP_PREFIX: &str = "voiced-pre-v";
 
 /// Highest schema version, i.e. the number of entries in `migrations()`.
-const LATEST_VERSION: i64 = 2;
+const LATEST_VERSION: i64 = 3;
 
 #[derive(Debug, thiserror::Error)]
 pub enum DbError {
@@ -169,8 +169,13 @@ CREATE TABLE IF NOT EXISTS invoice_drafts (
 );
 "#;
 
+/// Version 3: the user's default page setup (RenderPrefsV1 JSON; empty means built-in defaults).
+const M3: &str = r#"
+ALTER TABLE app_settings ADD COLUMN render_prefs TEXT NOT NULL DEFAULT '';
+"#;
+
 fn migrations() -> Migrations<'static> {
-    Migrations::new(vec![M::up(SCHEMA), M::up(M2)])
+    Migrations::new(vec![M::up(SCHEMA), M::up(M2), M::up(M3)])
 }
 
 /// Open (creating if needed) the database at `path` and bring it to the latest schema.
@@ -536,6 +541,22 @@ CREATE INDEX IF NOT EXISTS idx_invoices_created ON invoices(created_at DESC);
         assert!(backups_in(&backups).is_empty());
     }
 
+    #[test]
+    fn render_prefs_column_defaults_to_empty_and_round_trips() {
+        let conn = open_in_memory().unwrap();
+        let initial: String = conn
+            .query_row("SELECT render_prefs FROM app_settings WHERE id = 1", [], |r| r.get(0))
+            .unwrap();
+        assert_eq!(initial, "");
+
+        let json = r#"{"version":1,"templateId":"classic","pageSize":"LETTER","margins":"narrow"}"#;
+        conn.execute("UPDATE app_settings SET render_prefs = ?1 WHERE id = 1", [json]).unwrap();
+        let stored: String = conn
+            .query_row("SELECT render_prefs FROM app_settings WHERE id = 1", [], |r| r.get(0))
+            .unwrap();
+        assert_eq!(stored, json);
+    }
+
     fn build_v1_database(path: &Path) {
         let conn = Connection::open(path).unwrap();
         conn.execute_batch(V1_SCHEMA).unwrap();