feat: complete export/import cycle and remove sample data

Export now includes invoice_payments and recurring_invoices tables.
Import restored to use ID-based lookups and all fields for clients,
projects, tasks, and time entries. Added missing import support for
timeline_events, calendar_sources, calendar_events, invoice_payments,
and recurring_invoices. Export uses native save dialog instead of blob
download. Removed sample data seeding (seed.rs, UI, command).

src-tauri/src/lib.rs

@@ -6,7 +6,6 @@ use tauri::Manager;
 mod database;
 mod commands;
 mod os_detection;
-mod seed;
 
 pub struct AppState {
     pub db: Mutex<Connection>,
@@ -155,7 +154,6 @@ pub fn run() {
             commands::update_recurring_invoice,
             commands::delete_recurring_invoice,
             commands::check_recurring_invoices,
-            commands::seed_sample_data,
         ])
         .setup(|app| {
             #[cfg(desktop)]