Web: offer Reload to update when a new deployment is out (#231)

Every half hour the page fetches its own address past the browser's cache; when it loads
another content-hashed build of the app than the running one, the status bar offers "Reload
to update" (the page's unsaved-changes question still guards the reload). Rust only, no
service worker: the browser build has no other update path and none of the desktop's
download code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

apps/fieldcraft-web/src/deploy.rs

@@ -0,0 +1,75 @@
+//! A new deployment of the web app: every half hour the page's own address is fetched past the browser's cache, and when it loads another build
+//! of the app than the one running (Trunk names each build's files by their content hash), the
+//! status bar offers "Reload to update" (docs/updates.md). Reloading asks first when the model
+//! has unsaved changes, as closing the page does.
+
+use std::cell::Cell;
+use std::rc::Rc;
+
+use wasm_bindgen::JsCast as _;
+
+/// How often the page looks for a new deployment (seconds).
+pub const INTERVAL: f64 = 30.0 * 60.0;
+
+/// Watches for a new deployment.
+pub struct Watch {
+    /// The build this page runs.
+    running: Option<String>,
+    /// A fetch found another build.
+    found: Rc<Cell<bool>>,
+    /// App time of the last look.
+    pub looked_at: f64,
+}
+
+impl Watch {
+    pub fn start() -> Watch {
+        let html = web_sys::window().and_then(|w| w.document()).and_then(|d| d.document_element()).map(|e| e.outer_html()).unwrap_or_default();
+        let running = crate::build_in_page(&html).map(str::to_string);
+        if running.is_none() {
+            log::info!("deploy: no hashed build in the page (a development server?); not watching");
+        }
+        Watch { running, found: Rc::default(), looked_at: 0.0 }
+    }
+
+    /// Whether a newer deployment was found.
+    pub fn found(&self) -> bool {
+        self.found.get()
+    }
+
+    /// Fetch the page again and compare its build with the running one.
+    pub fn look(&self) {
+        let (Some(running), Some(window)) = (self.running.clone(), web_sys::window()) else { return };
+        if self.found.get() {
+            return;
+        }
+        let url = window.location().pathname().unwrap_or_else(|_| "./".into());
+        let init = web_sys::RequestInit::new();
+        init.set_cache(web_sys::RequestCache::NoStore);
+        let promise = window.fetch_with_str_and_init(&url, &init);
+        let found = self.found.clone();
+        wasm_bindgen_futures::spawn_local(async move {
+            let text = async {
+                let response: web_sys::Response = wasm_bindgen_futures::JsFuture::from(promise).await.ok()?.dyn_into().ok()?;
+                if !response.ok() {
+                    return None;
+                }
+                wasm_bindgen_futures::JsFuture::from(response.text().ok()?).await.ok()?.as_string()
+            };
+            // Offline or refused: look again next time.
+            if let Some(html) = text.await
+                && let Some(deployed) = crate::build_in_page(&html)
+                && deployed != running
+            {
+                log::info!("deploy: {deployed} is deployed (running {running})");
+                found.set(true);
+            }
+        });
+    }
+}
+
+/// Reload the page (the new deployment loads).
+pub fn reload() {
+    if let Some(w) = web_sys::window() {
+        let _ = w.location().reload();
+    }
+}