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/commands/invoice.rs

@@ -17,7 +17,7 @@ const INVOICE_COLS: &str = "id, number, series_id, invoice_date, due_date, clien
     tax_type, tax_rate, cgst_amount, sgst_amount, igst_amount, total, amount_in_words,
     bank_account_id, bank_snapshot, signature_path, notes, status, created_at, updated_at,
     doc_type, reverse_charge, COALESCE(vendor_snapshot, ''), snapshot_origin, cancelled_at,
-    cancel_reason, archived_pdf_sha256";
+    cancel_reason, archived_pdf_sha256, COALESCE(render_prefs, '')";
 
 fn map_invoice(row: &Row) -> rusqlite::Result<Invoice> {
     Ok(Invoice {
@@ -55,6 +55,7 @@ fn map_invoice(row: &Row) -> rusqlite::Result<Invoice> {
         cancelled_at: row.get(31)?,
         cancel_reason: row.get(32)?,
         archived_pdf_sha256: row.get(33)?,
+        render_prefs: row.get(34)?,
         items: Vec::new(),
     })
 }
@@ -922,6 +923,7 @@ mod tests {
             .query_row("SELECT render_prefs FROM invoices", [], |r| r.get(0))
             .unwrap();
         assert_eq!(prefs, "{\"theme\":\"plain\"}");
+        assert_eq!(inv.render_prefs, prefs);
     }
 
     #[test]

src-tauri/src/commands/settings.rs

@@ -6,7 +6,7 @@ use tauri::State;
 pub(crate) const SETTINGS_COLS: &str = "vendor_name, vendor_address, vendor_email, vendor_phone, vendor_pan,
     vendor_gstin, vendor_state_code, logo_path, signature_path, default_bank_id, default_tax_rate,
     default_tax_type, payment_terms_days, currency, onboarded, theme, gst_registration, default_hsn_sac, signatory_name,
-    signatory_designation";
+    signatory_designation, render_prefs";
 
 pub(crate) fn map_settings(row: &Row) -> rusqlite::Result<Settings> {
     Ok(Settings {
@@ -30,6 +30,7 @@ pub(crate) fn map_settings(row: &Row) -> rusqlite::Result<Settings> {
         default_hsn_sac: row.get(17)?,
         signatory_name: row.get(18)?,
         signatory_designation: row.get(19)?,
+        render_prefs: row.get(20)?,
     })
 }
 
@@ -54,7 +55,7 @@ pub fn save_settings(state: State<AppState>, settings: Settings) -> Result<Setti
              signature_path = ?9, default_bank_id = ?10, default_tax_rate = ?11,
              default_tax_type = ?12, payment_terms_days = ?13, currency = ?14,
              onboarded = ?15, theme = ?16, gst_registration = ?17, default_hsn_sac = ?18,
-             signatory_name = ?19, signatory_designation = ?20
+             signatory_name = ?19, signatory_designation = ?20, render_prefs = ?21
            WHERE id = 1"#,
         params![
             settings.vendor_name,
@@ -77,6 +78,7 @@ pub fn save_settings(state: State<AppState>, settings: Settings) -> Result<Setti
             settings.default_hsn_sac,
             settings.signatory_name,
             settings.signatory_designation,
+            settings.render_prefs,
         ],
     )
     .map_err(|e| e.to_string())?;
@@ -230,6 +232,26 @@ mod tests {
         (setting, flagged)
     }
 
+    #[test]
+    fn settings_carry_render_prefs() {
+        let conn = crate::db::open_in_memory().unwrap();
+        let read = |conn: &Connection| {
+            conn.query_row(
+                &format!("SELECT {} FROM app_settings WHERE id = 1", SETTINGS_COLS),
+                [],
+                map_settings,
+            )
+            .unwrap()
+        };
+        assert_eq!(read(&conn).render_prefs, "");
+        conn.execute("UPDATE app_settings SET render_prefs = ?1 WHERE id = 1", params!["{\"version\":1}"])
+            .unwrap();
+        let settings = read(&conn);
+        assert_eq!(settings.render_prefs, "{\"version\":1}");
+        let json = serde_json::to_value(&settings).unwrap();
+        assert_eq!(json["renderPrefs"], "{\"version\":1}");
+    }
+
     #[test]
     fn deleting_the_default_bank_promotes_the_lowest_remaining() {
         let mut conn = crate::db::open_in_memory().unwrap();

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();

src-tauri/src/models.rs

@@ -23,6 +23,9 @@ pub struct Settings {
     pub default_hsn_sac: String,
     pub signatory_name: String,
     pub signatory_designation: String,
+    /// The user's default page setup (RenderPrefsV1 JSON); empty means the built-in defaults.
+    #[serde(default)]
+    pub render_prefs: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -194,6 +197,8 @@ pub struct Invoice {
     pub cancelled_at: Option<String>,
     pub cancel_reason: String,
     pub archived_pdf_sha256: Option<String>,
+    /// RenderPrefsV1 JSON frozen at issue; empty when none was stored.
+    pub render_prefs: String,
     pub items: Vec<InvoiceItem>,
 }