v0.4.3: expose multi-script-ID support in the UI

The backend already supported a comma-separated list of Apps Script IDs
with round-robin dispatch and automatic 10-minute sidelining on quota
errors (429 / 403 / 'quota' in body). But the UI label just said 'Apps
Script ID' (singular), so users didn't realize multiple IDs are
allowed.

- Rename the label to 'Apps Script ID(s)'
- Hover tooltip explaining comma-separated input and quota auto-failover
- Placeholder hint 'id1, id2, id3 …'
- Live count line under the field when 2+ IDs are entered

Requested by a user in Farsi: 'add the ability to set multiple script
IDs; switch to the next when one hits its limit'. The feature existed,
this just surfaces it.

src/bin/ui.rs

@@ -291,11 +291,29 @@ impl eframe::App for App {
                 .num_columns(2)
                 .spacing([10.0, 6.0])
                 .show(ui, |ui| {
-                    ui.label("Apps Script ID");
+                    ui.label("Apps Script ID(s)")
+                        .on_hover_text(
+                            "One deployment ID, or several separated by commas.\n\
+                             With multiple IDs the proxy round-robins between them and\n\
+                             automatically sidelines any ID that hits its daily quota (429)\n\
+                             or other rate limits for 10 minutes before retrying it."
+                        );
                     ui.add(egui::TextEdit::singleline(&mut self.form.script_id)
+                        .hint_text("id1, id2, id3 …")
                         .desired_width(f32::INFINITY));
                     ui.end_row();
 
+                    let id_count = self.form.script_id
+                        .split(',')
+                        .map(|s| s.trim())
+                        .filter(|s| !s.is_empty())
+                        .count();
+                    if id_count > 1 {
+                        ui.label("");
+                        ui.small(format!("{} IDs — round-robin with auto-failover on quota", id_count));
+                        ui.end_row();
+                    }
+
                     ui.label("Auth key");
                     ui.horizontal(|ui| {
                         let te = egui::TextEdit::singleline(&mut self.form.auth_key)